Mob

Mobs are the various types of creatures and monsters found in Minecraft. Currently, there are 14 unique mobs although alternative mobs can be implemented by mods. Mobs are affected by the same environmental hazards as the player, including burning and collision (falling) damage, with the exception of chickens, who do not suffer collision damage. Mobs can see you up to 16 blocks away, except for the Ghast which can see you 100 blocks away. Up to 200 hostile mobs and 15 neutral mobs can be on a map at a time. Mobs spawn no closer than 24 blocks from the player.

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 14 types of mobs: Wolves, Pigs, Sheep, Cows, Chickens, Squid, Zombie Pigmen, Spiders, Zombies, Skeletons, Creepers, Slimes, 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, with the exception of Squid which spawns in water blocks. in beta 1.8 passive mobs will flee while being hunted

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 and Slimes.

Note: All mobs' hostility levels are based on how they are spawned by default(except for a tamed wolf). 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.*A "scary monster" is planned to appear in the 1.8 update.


 * A new mob texture was found with Beta 1.7, titled silverfish.
 * 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 1.8. Notch also said something similar in an interview.
 * Notch said he was working on a new mob after drinking some coffee. Later was confirmed as the new Endermen mob.

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.

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.