Enderman

An Enderman (plural Endermen) is a 3 block-high, humanoid mob with long arms and legs, glowing purple eyes and a purple particle effect similar to a Portal.

Endermen will not attack the player unless one points his/her crosshair directly on them. In creative mode, they will only attack you if you attack them, similar to wolves, and all other hostile mobs. They are rare in comparison to other mobs, but can still be seen regularly at night on the Overworld in groups of up to five (previously eight). Their two iconic abilities are the ability to pick up and move blocks, and to teleport. When teleporting, an Enderman will leave a faint trail of particles leading to itself from where it teleported. Endermen will take damage from coming in contact with water (including water blocks and rain).

As of Minecraft full version 1.0 Endermen teleport when coming in contact with rain, water, or sunlight - though not if set on fire with a flint and steel or by contact with lava.

Endermen are not hostile until the player looks at them. In survival single player "Looking at an Enderman" is defined as aiming the crosshair on the Enderman's head or torso. In SMP, if you look at the enderman's head/eyes, it won't be alerted. However, if you look at its body or upper legs, it will become aggressive.

Endermen frequently spawn in the End, their homeworld, but they still require a light level of seven or less in order to spawn.

Moving blocks
Endermen will pick up and move certain blocks, regardless of whether they were natural or placed by the player. They will pick up blocks horizontally and vertically nearby, within a short reaching distance similar to the player's, at heights from just below their feet to just above their head (5 layers).

They will not drop the blocks when killed - the blocks disappear.

Since Beta 1.9 Pre-release 2, Endermen can only pick up the following blocks:

Endermen cannot pick up entities, such as Boats, Minecarts, primed TNT, and mobs.

Damage to structures and environment
Because Endermen possess the ability to move blocks, they can cause damage to the natural environment and in some cases player-built structures.

On the overworld signs of Endermen activity will become apparent in frequently loaded chunks in the form of misplaced blocks. In desert areas, Endermen may also pollinate cacti by removing blocks and placing them on fresh sand, gradually increasing the population of cactus in frequently loaded deserts.

Endermen do not spawn in lit areas, but they may wander or teleport into lit areas from adjacent dark ones, particularly if rain or the rising sun causes them to teleport, so simply lighting structures liberally with torches is not a guaranteed defense.

Preventing Attacks
The only way to get an Enderman to chase you is to hit it, or look at it with the crosshair. As Endermen are 3 blocks tall, you can keep your house's interior 2 blocks tall to prevent Endermen from teleporting into your house.

If the player sits in a pool of water, the Enderman will take damage from the water and then teleport away in response to the damage, and become neutral.

If the player wears a pumpkin on their head or looks at an Enderman through a transparent block such as glass, ice, leaves, or fence posts, it will not turn hostile when looked at. Once provoked, however, wearing a pumpkin will not prevent them from attacking or teleporting.

Killing Endermen
Endermen are damaged by attacks, fire, lava, water, and poison.

You can perform preemptive strikes against Endermen by keeping your crosshair over them, which will cause them to stay still for several seconds, or until the enderman is hit, whichever comes first. A player can also get in the first hit by looking only at the enderman's legs until they attack - though once hit the enderman will become aggressive as normal.

Endermen that wander into fire or lava will not teleport away, nor will they become aggressive to the player. This can be used to weaken or kill them, though the second will require that they be set on fire twice (likely by placing fire with a flint and steel) due to their high hit points.

Endermen cannot be killed by player use of a bow. Arrows, snowballs, and eggs will not hit them due to the fact that they usually teleport away before impact - Because they do not get hurt projectiles will not cause Endermen to become hostile. If a projectile does manage to somehow damage an enderman, such as an accidental arrow hit from a skeleton while they are attacking the player, they will become hostile to the source of the damage.

Hostile Endermen will teleport away after a player hits them, though they may attack first. Similarly, Endermen may teleport away several seconds after attacking. When this happens in a small underground space, the Enderman may teleport into inaccessible caves or to the surface and may or may not teleport back. The Endermen may also teleport behind the player so caution is recommended. Endermen teleportation can be tracked by looking for the glowing teleportation trail they leave behind.

Another effective way of killing an Enderman is to make a 4-block high pillar underneath yourself, then look at an Enderman. The Enderman will run toward you and come into melee range. You will be in range to attack them, but they will not be in range to attack you. Make sure there are no skeletons nearby (within 16 blocks), as they will shoot you off your pillar and make you a vulnerable target.

If you can trap an Enderman in a minecart, he will be unable to teleport, so you can put a water source block above him to drown him.

A quick, effective way to kill an Enderman is to quickly run towards one with or without a weapon (and without looking at its head), attack its legs once, and have four or more tamed wolves to finish it off.

A different approach to killing an Enderman is to make a bunker with an 2 block high interior with a 1 block slit to look out of. You can then provoke the Enderman by looking at it, the bunker will prevent an Enderman from teleporting to you, while letting you attack at the legs of the Enderman without getting hit. This also works in cave systems, if you should find an Enderman lurking inside.

Another quite effective way to kill an Enderman is to set the ground on fire in front of it, and as it walks around set the ground it walks on on fire. It will not attack you, and it may be one of the only ways to get Ender Pearls without being attacked or have the Enderman teleport away.

History
But they will drop diamonds again as of snapshot 12w06a.
 * The Enderman was introduced in Beta 1.8, part one of the Adventure Update.
 * Endermen used to burn in sunlight, but as of 1.9 pre-release 4, Endermen won't burn in daylight. They will instead teleport away when the player walks up to them in the daytime, making it extremely hard to get close enough to kill them.
 * At first, when Notch was still designing the Endermen, he thought they were not 'creepy' enough, coming with the idea that having a situation which the player would want to avoid could make the Enderman significantly more scary. This gave Notch the idea to implement the staring feature where aiming the crosshairs on the Enderman's face would cause a suspense in which breaking eyecontact would trigger an attack, as well as the Enderman having the ability to teleport so that when the player has stared at the Enderman they will have to face a consequence for that action.
 * In the Beta 1.8 demo at PAX, Endermen dropped diamonds as a placeholder.
 * In Beta 1.8 pre1&2, Endermen originally had green eyes and emitted black smoke. The release of Beta 1.8 changed the eyes to purple and black smoke has been replaced with purple particles similar to the particles emitted from Nether portals.
 * In Beta 1.8 Pre-releases, Endermen were able to pick up any block, including Bedrock.
 * Before the Sound Update, Endermen used zombie sounds as a place holder. On November 13th, 2011 Endermen did receive their own audio scheme, most notably a long moan overlaid with static for while the player stares at them.
 * As of Beta 1.9 pre4, the Endermen have a realm of their own: The End, where they spawn frequently.
 * Notch stated "Endermen suck because they're annoying, and because I nerfed them and made them too easy. I shall fix this."
 * Because of this, Endermen were modified in Beta 1.9 pre3 to have 40 hp (instead of 20), requiring 6 hits from a diamond sword to kill. Combined with their teleportation aspect, Endermen are more challenging.
 * Before Beta 1.9 pre-release, Endermen could be killed with the use of a bow. As of all the pre-releases, arrows, snowballs, and eggs will not hit them due to the fact that they teleport away before impact.
 * Since 1.9 pre-release, if you were to pour water on a hostile Enderman, the Enderman will teleport away and become neutral again.
 * Endermen in the first snapshot used to have grey eyes

Trivia

 * The exact size of an Enderman is 0.6x0.6x2.9 blocks.
 * Endermen cannot teleport when they are in minecarts.
 * If a Skeleton shoots an Enderman when attempting to shoot the player, the Enderman will completely stop chasing the player even after it has killed the offending Skeleton.
 * A similar nature can be encountered when an Enderman is hit by an Enderdragon; They will ignore the player and chase after the offending Enderdragon, should they survive the attack.
 * Endermen located in the End do not pick up Endstone.
 * The name "Enderman" is a reference to the Internet meme known as "the Slender Man", a similar-looking fictional cryptid. The name was given to the mob by Notch in a Minecraft Reddit thread.
 * The audio for Enderman are a slight nod to the video and audio disrupting nature of the above mentioned Slenderman, and their scream sounds much like that of the Slenderman.
 * Endermen have the ability to open their mouths. In their texture, their jaw is separated from their skull and in the 1.8 Official Trailer, an Enderman attacked the camera with its mouth open. This makes the Enderman the first mob with the ability to physically open their mouths without simply switching to a different texture. They only open their mouths when they attack. Due to the player's perspective, the player sees the back of the Enderman's head when they open their mouths. If a player is one block higher, they see through the Enderman's open mouth. They open their mouths by moving their skulls higher than their jaw.
 * Endermen's eyes glow in the dark, like Spiders', and Endermen's full body glows in the fog, also similar to a Spider's body.
 * Endermen spawn in the rain, but will continuously teleport and take damage until they find shelter.
 * Like every other creature in the game with the exception of Spiders, Endermen can't see the player through transparent blocks such as Glass.
 * Since the 1.9 pre2 Endermen will teleport away whenever they are touched by water, when they would be hit by a player's projectile, or when exposed to daylight.
 * Endermen can teleport very far, even when moving several hundred kilometres from them they will continue to follow until killed or distracted by exterior elements such as water or sun.
 * It is unknown if there is a limit to the number of Endermen that can follow a player, but as shown here, 30 is not the maximum amount that can follow.
 * An effective way to gain easy experience is to go the End, make a 1x1 tower 4 high, stand on top of it, look at Endermen and have them teleport to you while you slaughter them. They cannot reach you because the tower is too tall.
 * Even in an aggressive state, they can still pick up blocks, which at times makes it seem as though they are tearing through walls to reach the player.
 * When an Enderman picks up a flower or a normal mushroom, those blocks will appear to be floating between their grasp. This can lead to the player being beaten to death with a flower.
 * Even though water/rain is harmful to them, it will still put out a burning Enderman.
 * Endermen are the only current mobs which spawn naturally pan-dimensionally, appearing both in the Overworld and the End. Zombie Pigmen can be created in The Overworld if Lightning strikes a Pig, Snow Golems can be made by the player anywhere, and chickens can be hatched from an egg anywhere - but these creatures do not naturally spawn.
 * Contrary to popular belief, Enderman are neutral mobs, not hostile. Most people think they are hostile because they spawn in the dark, do not spawn in peaceful, and sometimes beat the player to death.
 * Endermen continue to shake when provoked even while the game is paused.
 * Endermen can pick up blocks even if they are covered by Paintings.

Bugs
for finding stuctures such as shafts
 * In SMP, Endermen don't open their mouths or shake when aggressive.
 * When using a custom texture pack, Endermen will be bright white overlapping their texture. This is caused by the blank spaces of their enderman_eyes.png file. If you create a custom texture pack you must make sure that you only change the 6 pixels actually used by their eyes. An alternative would be to use the original png from the minecraft.jar in your pack. Using the most recent release of MCPatcher also fixes this bug. This can also be avoided by making the enderman_eyes.png translucent, even only slight translucency will fix this.
 * If a hostile Enderman is attacked by another mob and kills the attacker, it resets to being neutral towards the player.
 * If an Enderman picks up a flaming block (i.e. Wood, Netherrack, etc.) and then places it again, the block will be extinguished, but the Enderman will be on fire.
 * In Minecraft 1.0 Enderman sometimes teleport to the surface then immediately teleport away, possibly as a result of touching water underground.
 * If you quit a game while being attacked by a hostile Enderman, and re-join the game, the Enderman will return to being neutral.
 * If caught with a fishing rod while neutral, an Enderman will remain neutral even after being pulled in, though the fact that they teleport away when hit with the fishing lure makes this of limited potential use.
 * When an Enderman picks up TNT, the TNT block's texture is inverted (Notice the "N" in TNT is reversed(ТИT)).
 * If you fire an arrow at an Enderman who is in a mine cart, it will teleport away and reappear in the same mine cart. Also, at times it will turn red as if it had taken damage, but it will not die from arrow shots.
 * If seen through the mouth of an enderman you can see underground structures making them a good tool
 * When it rains and an enderman is inside the space of a fence block, it won't teleport away.
 * When you shoot an Enderman with a bow enchanted with flame, the Enderman teleports, but it is on fire.
 * And a really rare bug is the Enderman spawing out of a Nether portal and dying.