Enderman

The Enderman is a mob that was introduced in version 1.8; The Adventure Update. The Endermen are 3 blocks high black creatures with long arms and legs,as well as glowing purple eyes, and they emit purple particles similar to a Nether portal. They are rare in comparison to other mobs, with the exception of the Cave Spider. They also have the ability to pick up and move blocks around. They are generally neutral. However, if a player looks at them by placing his or her crosshair directly over them for half a second, they freeze, stare back at the player, open their mouth and become hostile. This only happens, however, if the player is not farther than 64 meters from them. They stay frozen as long as the player doesn't move the mouse at all, even if just one pixel. When the player moves the mouse (regardless of whether it's still pointing at the Enderman), they will quickly run towards the player; if they are a long distance away, they will teleport several times until close enough to reach the player. Endermen can teleport about once per second, and when they get close enough they teleport behind you, so beware. Endermen take damage from coming in contact with water (including water blocks and rain) and sunlight. Their exact size is 0.6x0.6x2.9.

Preventing Attacks
It is important to note that the Endermen will become aggressive if players attack them. It is safe to look at them through transparent blocks like glass, glass panes, iron bars, ice and vines - but not stacked half slabs.

Endermen will not attack players who are wearing a pumpkin. A good idea is to make a texture pack with no pumpkin blur.

It should also be noted, that looking at an Enderman's legs or head will not stop them from attacking. The player must look at the torso to prevent further attacks.

Further, an Enderman in a minecart seems to be incapable of attacking.

Killing Endermen
Despite their appearance, Endermen are easier to kill than most other mobs, since they are damaged by water, sunlight, fire, lava, rain and attacks alike. Emptying a bucket of water on the ground will damage the Enderman without it becoming hostile to you. Arrows and all types of sword are effective against one, though wood and stone swords may not deal enough damage to kill the Enderman before it kills you.

Hostile Endermen will teleport away when the player closes in on them to strike. When this happens in a small underground space, the Enderman will teleport into inaccessible caves or to the surface. They will not teleport back to the player unless he moves to an area that has enough space for the Enderman to teleport back. On the surface, an Enderman's tendency to teleport away when the player is close enough to attack can be used to keep an attacking Enderman at bay.

When killed, regardless of the method of death, any blocks held by the Enderman disappear. They do not drop or get placed.

Moving blocks
Endermen will pick up and move both natural and player-placed blocks. They will pick up blocks horizontally nearby within a short reaching distance similar to the player's, at heights from their feet to their head (3 layers).

The source code of MobEndermen indicates they will only pick up blocks of the following types:

Player testing has indicated that they cannot pick up the following blocks:

Damage to Structures
Because Endermen possess the ability to move blocks, they can cause damage to player built structures by breaking and replacing blocks. The resulting holes can let in other mobs in addition to being simply unsightly.

Common ideas for constructing Enderman-proof structures have been: Notably the Endermen can reach through blocks to grab blocks behind so put anything important 1 block in if using vines, or two blocks in otherwise, or make walls multiple blocks deep.
 * Cover exterior walls with vines, so that the Endermen can not reach the blocks underneath.
 * Build walls out of materials Endermen can't pick up, such as stairs.
 * Surround the structure with a moat to kill the Endermen. Either lava or water can be used.

Enderman do not spawn in lit areas, but they may wander into lit areas from adjacent dark ones, so simply lighting structures liberally with torches is not a guaranteed defense.

Bugs

 * In SMP, endermen do not charge if you look at them. However, their vision bounding box is off, so if you look above the endermen, they will become aggresive.
 * If an Enderman picks up a block with a tile entity, such as a chest, its contents will be deleted.
 * In survival mode, Endermen can pick up and place bedrock.
 * 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.
 * Endermen appear to spawn in well lit areas.
 * Endermen do not appear to be as rare as they were originally intended to be. They can often be seen in groups of up to 5.
 * When your crosshair is on the Enderman, and it is still frozen, you can hit it from an extended distance than normal reach.

Trivia

 * 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.
 * 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 Reddit thread.
 * Due to the similarities to the Slender Man, many have petitioned to change the name "Endermen" to "Far Landers". Notch has stated however that he will not change their name, regardless of this petition, going as far as to joke that he should change the name of the Far Lands to "The End" instead.
 * At the PAX Beta 1.8 demo, Endermen dropped diamonds; in the actual release they have a chance to drop an Ender Pearl.
 * 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 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.
 * As of now, enderman sound like zombies, but that is a place holder; new sounds were made, but not yet implemented.
 * Endermen's eyes glow in the dark, like spiders'.
 * Endermen's full body glows in the fog. (Like Spiders)
 * Endermen seem to shake while you look at them.
 * When you pause the game, the Enderman will keep shaking.
 * What Notch wrote in his first official description of the Endermen, "-the feeling of control", may have meant more than it appeared, referencing a deeper fear in players, the fear that they now had something closer to an equal, and thus something to challenge their control.
 * Endermen seem to have a smiley face pattern on the back of their heads.
 * Contrary to popular belief, there are no other eye colours besides purple. The mob/enderman_eyes.png file in minecraft.jar only has one colour, and what looks like what eyes in mob/enderman.png is actually the spot where the eye texture gets overlayed, to make the eyes glow like spiders'.
 * When an Enderman picks up a block, it makes the sound of that block type being destroyed.
 * Endermen are quite common above ground, as stated above, spawning in groups of sometimes about 5, while below ground they are a bit rarer and cannot spawn in low roofed areas, such as confined caves. It is not reccommended to run underground to get away from them as they may still spawn and will chase you if provoked.
 * In the 1.8 pre-release, Enderman had glowing green eyes instead of purple.
 * Endermen do not appear to spawn while it rains.