Mob

Mobs are living, moving game entities. Generally, mobs are affected by the environment in the same ways as the player—they are subject to physics, they can be hurt by catching on fire, falling, drowning, or suffocating, and they can even ride minecarts. When mobs are killed, they drop items which may be useful resources.

Mobs come to exist by spawning in various ways. Players can spawn most mobs using Spawn Eggs in Creative Mode.

Mobs are aware of players within 16 blocks of them, except for Ghast, which can see up to 100 blocks away and Endermen, which can see up to 64 blocks away.

The term "mob" is short for "mobile" and has been used as a general term referring to any moving entities in games since the first MUDs surfaced.

Current Mobs
There are 26 mobs in Minecraft 1.2.5, excluding the player.

Passive

Passive mobs will never attack the player.

All passive mobs require land and light to spawn on. However, squid are the only mob that requires water to spawn.

Neutral

Neutral mobs will not attack the player unless provoked. The act of provoking neutral mobs varies between mobs.

Hostile

Hostile mobs will attack the player when in range.

Cave Spiders, Zombies, Creepers, Skeletons, Spiders, and Spider Jockeys require a light level of 7 or less to spawn.

All mobs' hostility levels are based on how they are spawned by default. Spiders may become neutral, but they do not spawn neutral.

Utility

Utility mobs are created by and serve the player.

Bosses

Boss mobs have more complicated attack patterns and movements. They also have a large amount of health as well. Boss mobs only spawn once per world.

Planned, Unused, Removed, and Potential Mobs
Planned

Planned mobs are speculated to be added in a future version of Minecraft.

Unused

Unused mobs are found in the source, but are not being used at this time. Through hacking, it is still possible to spawn them. THE VINEMAN DOES NOT APPROVE

Removed

Removed mobs have been removed from the game and can only be found in previous versions. DO NOT ENTER THE JUNGLE>

Potential Future Mobs
There are hints that new mobs might be released in the distant future. Below is the list of mobs that have been mentioned at least once by Notch, Jeb, or other Mojang employees who work on Minecraft and have a possibility of appearing in the game. THE VINEMAN DOES NOTT APPROVE
 * Aside from zombie pigmen, living pigmen have been confirmed to appear in a future update. However, they are not being currently worked on.
 * In the Halloween Update, Notch said he would add fish. However, he had limited time, and was thus only able to add them as an item. There is coding for fish in the game, but no model exists at this moment. Like Pigmen, Mojang is not currently working on adding fish.
 * Notch confirmed during the Brocraft.net stream that he will be adding prefix mobs. This means that there will be mobs such as 'Burning Spider' 'Giant Zombie' 'Cobble Creeper', etc. It is not known if these are still planned to be in a future update. HoDONT ENTER THE JUNGLE>wever, the dreaded Cave Spider and technically the Charged Creeper are prefix mobs. The Magma Cube, also known as a Lava Slime, and Mooshroom, referred by the game as 'Mushroom Cow', may also be prefix mobs. Although there currently are not any Pigmen in the game, the Zombie Pigman and Giant, the aforementioned 'Giant Zombie,' may have been prefix mob prototypes. Now, with the new Iron Golem, there is another example of a prefix mob in regards to the Snow Golem.

Movement
Mobs will ordinarily wander around at random and usually avoid walking off cliffs high enough to cause falling damage, though they can be fooled by non-solid blocks such as signs. In this state, mobs (besides Ghasts and Endermen) have a search radius of 16 blocks.

Mobs are capable of climbing ladders just as the player is: if they try to move towards a ladder they're touching they move up it instead. However, mobs do not see ladders nor do they purposely pathfind up them, so they will not attempt to avoid falling off a ladder should they be on the edge of one and they did not climb all walls. Spiders and Cave Spiders can climb all blocks as if they were ladders.

Mobs will stop wandering within 5 seconds if there is no player within a 32 block radius. In this state, they will glance around randomly, but they won't walk anywhere. They can still be moved by other means such as flowing water, minecarts, etc.

As of Minecraft 1.2 the AI (artificial intelligence) received several updates. Including a path finding update. Here's a video showcasing the AI's path finding.

Hostile Mobs
If the closest player to a wandering hostile mob is within the search radius of 16 blocks and a line of sight can be drawn, (Spiders can always draw line of sight) the mob will enter pursuit mode. In this mode, mobs can see through all blocks and will walk towards the player. Some mobs, most notably skeletons and creepers, will start circling the player (moving to the left from the player's view) when close enough (skeletons at a long range for strafing, creepers at a very short range). As of the latest weekly build zombies will sink in water and whilst in pursuit mode can path around walls to get to the player.

Passive Mobs
Animals are attracted to both light and grass. If there is a lot of grass nearby, they will wander toward it regardless of light level. If they are completely surrounded by grass, they will wander aimlessly. If they can't see any grass, they will wander toward light.

Passive mobs will flee randomly when attacked.

Passive mobs will follow a player if their currently held item is wheat. As long as the player does not get too far ahead of the mob, they will follow them over long distances. This can be used to corral mobs into pens in order to make farms.

If wheat is given to the mob by right clicking the mob, the mob will enter breed mode. See main article Breeding.

No passive mob, except for squid and ocelots, will despawn if you move too far away from them. This also applies to wolves.

VINEMAN
VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMANVINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMANVINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN VINEMAN

Trivia

 * The Halloween Update version 1.2.0 (Alpha) allowed hostile mobs to spawn in higher light levels at lower depths, using the formula 16 - (Layer ÷ 8). At level 8 and below, mobs could spawn even in sunlight. Notch reverted mob spawning to the original method in version 1.2.1 (Alpha) saying, "It was far too annoying. I have plans on what to do with this."
 * When mobs spawn, smoke comes out of them, but not in multiplayer.
 * Mob noises can be heard up to 16 blocks away which is the same distance that hostile mobs can sense you. Therefore, if you hear a hostile mob, it is highly likely that they have already noticed you.
 * Players can approach a hostile mob without it noticing the player if they ensure that there are solid blocks between them and the mob.
 * After killing a player, or if a player dies while hostile mobs are pursuing the player, the hostile mobs will gather around the player's deathplace, attempting to attack the player. This is because the player has been made invisible and non-solid, but still exists in that position. (evident by the "Game Over" screen still remaining in that position). The hostile mobs' heads will "spazz out", a humorous glitch caused by the pathfinding of the mobs so it looks like the hostile mobs are eating the player. A player can use this to their advantage by holding a large number of hostile mobs over their deathplace while another player may pass by unnoticed. This quirk can also work against the player in the case of death by ghast, since the ghast will continue to fire at the player, possibly destroying more of the player's constructions or dropped items. The targeting of the player by the hostile mob is canceled once the player logs out, quits, or respawns.
 * If a hostile mob has found you and activated hostile mode, you can separate you and the mob through a wall, relog, and the mob will no longer be in hostile mode.
 * When a hostile mob is attacked by another one, it will attack the attacking mob. For example when a skeleton arrow hits a zombie, the zombie will attack the skeleton.
 * Mobs still spawn on peaceful, but disappear too quickly for the game to render their sprites.
 * All mobs spin when on fence blocks.
 * Mobs taller than a block in a minecart will seem to be on top of a transparent block if the block is above to rail the cart is on and play the moving animation, this can let you "control" the mob with powered rails, producing a humorous effect (Further testing required)