Mobs are non-player characters (NPCs) which take the form of clones of the default skin, created by pressing 'G' whilst playing in single-player mode.
If the player has a custom skin, any mobs created will still be of the default skin, and not assume the look of the player's skin.
They cannot create or delete blocks, but just move around the map randomly. They respond the same as the player would to in-game conditions, in that they are affected in the same way by blocks as the player. However, they move with a constant speed, no matter if they are submerged in fluids or not.
Types of mobs
Currently, there are 5 types of mobs:
A human, the default mob of Minecraft.
Humans
Humans are non-player characters (NPCs) which take the form of clones of the default skin, created by pressing 'G' whilst playing in single-player mode.
If the player has a custom skin, any Humans created will still be of the default skin, and not assume the look of the player's skin. They cannot create or delete blocks, but just move around the map randomly. They respond the same as the player would to in-game conditions, in that they are affected in the same way by blocks and fluids as the player. They cannot be spawned in survival mode.
Pigs
A pig, aimlessly wandering
Pigs are neutral mobs that appear in survival mode. Pigs exist for the express purpose of providing health replenishing food to the player. Pigs spawn in groups of three or four and wander aimlessly around a map until they are killed. Though not hostile, pigs can bump around the player. As of now, Pigs drop one or two health restoring mushrooms when they are vanquished due to time restraints in designing meat.
Skeletons
A fearsome skeleton
a Skeleton is a hostile mob found in survival. Skeletons are spawned randomly in new maps and will follow players when approached much like a Zombie
Attributes
Skeletons take between 4 and 6 hits to kill making them one of the more durable enemies, and inflict 2 hearts of damage upon attack. Skeletons can wear armor. As of the patch of September 3rd, Skeletons can shoot arrows, something that can be exploiting by the player to cause in-fighting, which was also adding in the patch. Killing them results in arrows being fired radially from the corpse to be collected by the player.
Zombies
A Zombie, poised for attack
The zombie is a hostile mob that appears in Survival and Zombie Siege.
Attributes
As of the September 3rd patch, Zombies are the fastest mobs in the game. Zombies can navigate and will chase players. Their attack deals 3 hearts of damage, making them a bit of a threat. They're usually seen in packs of two or more. Zombies can sometimes be found wearing one piece of armor or the whole set.
Creepers
The Creeper, almost dead.
The Creeper is a type of mob, found in Survival and possibly Zombie Siege.
Attributes
While his attack only does 2 heart damage instead of 3, he behaves the same as a zombie. After taking some damage, he will start flashing, showing he is ready to detonate. Upon being killed, he creates an explosion, which can damage the player and creates a small crater, destroying plenty of blocks. Unlike the zombie, he cannot obtain armor. In groups they can cause a chain reaction of explosions, which can be very dangerous to the player. They can also be used for quick mineral mining, if the player leads them into an area populated with gold. They also pose a problem, because when multiplayer survival is released, they may be used for griefing, although some people may consider this a usefull and legitimate way of attacking enemy players.