Mob

Mobs are living, moving game entities, such as animals and monsters. 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.

Mobs come to exist by spawning in various ways.

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

About
As of Beta, there are a total of 17 types of mobs: Wolves, Pigs, Sheep, Cows, Chickens, Squid, Zombie Pigmen, Spiders, Zombies, Skeletons, Creepers, Slimes, Endermen, Silverfish, Cave Spiders, Spider Jockeys and Ghasts. More information is listed below.

Current Mobs
Passive

Passive mobs will never attack the player.

Require a grass covered block to spawn and a light level of 9 or higher, with the exception of Squid which spawns in water blocks.

Neutral

Neutral mobs will not attack the player unless provoked.

Aggressive

Aggressive mobs will attack the player when in range.

Require a light level of 7 or less to spawn with the exception of Ghasts, Silverfish and Slimes.

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

Unused, Removed, Planned, and Potential Mobs
Unused

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

Removed

Removed mobs are mobs that have been removed from the game and can only be found in previous versions.

Planned

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

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 by Notch at least once and have a possibility of appearing in the game.


 * Aside from zombie pigmen, living pigmen have been confirmed to appear in a future update.
 * 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.
 * Dragons - Notch hinted during the Spider-Jockey update that he will add Dragons to the game. Dragons may also be able to fly or breathe fire. Notch confirmed on his Twitter that he will definitely add a form of flying transportation. There is a chance that Dragons may be able to be ridden in a similar manner to pigs.
 * 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.
 * Jeb recently posted that he likes the idea of players deciding when to summon the boss mobs like in Terraria, which means that boss mobs may be added in 1.9. Notch also said something similar in an interview.
 * A new species of cow, the Mooshroom, which will probably spawn in mushroom biomes in 1.9.

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 the Ghast) have a search radius of 16 blocks.

If the closest player to a wandering aggressive 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).

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. Spiders can climb all blocks as if they were ladders.

In beta 1.8 passive mobs will flee randomly once attacked. After seeing this code in action, Notch has been inspired to write more mob interaction code for the future.

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.