Mobs are various different types of monsters found in survival.
Types of mobs
Currently, there are seven types of mobs. Each of these mobs is affected by the same environmental hazards as the player, including drowning, burning and fall damage.
All mobs disappear after a while if the player gets out of their radius.
A human
Past Mobs
Humans
Humans are non-playable characters (NPCs) which take the form of clones of the default skin. They can be created by pressing 'G' while playing in Creative mode. It is not possible to spawn humans in Survival mode. If the player has a custom skin, any humans created will still use default skin and will not assume the look of the player. Humans cannot create or delete blocks, they just move around the map aimlessly. They are affected by blocks and fluids the same way the player is. Humans currently do not appear in Survival or indev. The human model was originally going to be used for a top-down game called "Zombie Town" by Notch. The ability to spawn these mobs has currently been disabled by Notch, purportedly to prepare for future updates.
Rana
"Rana" were mobs who resembled little girls with frog hats. They were originally in-game as a test; Rana was made by "Dock", Minecraft's artist, for another project of his.
Current Mobs
A pig, aimlessly wandering
Pig
The pig is a neutral mob that appears in Survival, Infdev, and Indev 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 they are not hostile, pigs can bump into the player, possibly pushing the player over a cliff to his death. As of now, in Indev Pigs drop anywhere from zero to two units of pork when they are vanquished. In Survival they drop mushrooms, as pork has not been added to Survival mode yet.
A fearsome skeleton
Skeletons
The skeleton is a hostile mob found in Survival and Indev. Skeletons are spawned randomly in new Survival maps and follow players when approached, much like zombies. In Indev mode, they only spawn in dark areas, such as unlit areas on the surface at night, or in unlighted caves.
Skeletons are vastly different in Survival mode than in InDev. Both variations can, however, fire on other mobs (of the same or different type).
In Survival, skeletons are similar to zombies. They take 4-6 hits to kill and can wear NPC armour, although the armour does not give them any additional hit points. They inflict 2 hearts of damage upon attack. Skeletons will fire arrows at you and, upon death, will burst out arrows that the player can collect. Killing said skeleton will net you 120 points.
In Indev, skeletons are fairly fast-paced enemies that will shoot arrows at the player at a decent speed. Unlike the skeletons in Survival mode, InDev ones do not currently wear armour. A skeleton can take 3-6 hits with a Stone Axe. They randomly drop 0-2 arrows upon death. If touched by sunlight (the sun at a 15 degree angle or greater) InDev skeletons will begin to take burn damage until death.
A zombie, poised for attack
Zombies
The zombie is a hostile mob that appears in Survival and Zombie Siege. As of the September 3, 2009 update, zombies are the fastest mobs in the game. Zombies can navigate and chase players. Their attack deals 3 hearts of damage. They usually appear in packs of two or more. In Indev mode they only spawn in dark areas, such as unlit areas on the surface at night, or in unlighted caves. Zombies can sometimes be found wearing one piece of armor or the whole set. When a zombie is killed, the player gets 80 points. They are much slower in Indev. Like the Indev skeletons, they burst into flames if they come into contact with direct sunlight.
In the free version of Survival mode, armored zombies appear randomly along with groups of normal zombies. These zombies are easily distinguishable by their body armor and/or helmets. Their armor does not however give them any additional hit points, the difference is purely cosmetic.
Two creepers, the left one is about to explode.
Creepers
The creeper is a type of mob, found in Survival and Indev. Like the skeleton, the creeper differs heavily from one version to another. The creeper design is based on a failed attempt to make a pig model, hence the four legs.
Survival: The creeper behaves much like a zombie, except its attack deals only two hearts worth of damage instead of three. After taking some damage, it flashes, showing it is about to detonate. Upon being killed, it causes an explosion which can hurt the player, and leaves a small crater, destroying plenty of blocks. Currently, their explosions have no effect on rock, cobblestone, ore, or metal tiles. Unlike the zombie, it does not wear armour. In groups, they can cause a chain reaction of explosions, which can be very dangerous to the player. The creeper is worth 200 points.
Indev: The creeper is a slow-moving kamikaze mob. Upon getting close enough, it will let out a "burning fuse" sound and detonate after 3 seconds. This count-down can be halted, but whether it stops or not seems to be random, or possibly linked to the amount of time the "fuse" has been burning. Killing the creeper before this countdown starts or finishes will not cause the creeper to detonate. The creeper will cause a 4x4x4 spherical crater upon detonation. Unlike in Survival mode, the explosion caused by a creeper in Indev mode can destroy stone and other hard materials. Creepers do not catch fire when daylight comes. It drops 0-2 units of gunpowder when killed.
A spider
Spiders
Spiders The spider was the second mob added to the game. Unlike the zombie (1x1x2) the spider is a flat mob (2x2x1). It cannot enter the same narrow spaces that zombies and other tall mobs can, but it can crawl into places where the ceiling is too low for any other mobs.
Survival: The spider is basically a zombie that can jump farther. Seeing as they are much larger than zombies, they can be hit rather easily with arrows. A lone spider is easy enough to deal with, but a pack can quickly close distance on you due to their jumping ability. Killing this mob nets 105 points.
Indev: In Indev mode the spider is drastically different from the zombie. For starters, their eyes glow red and it has multiple model skins. It moves at the same speed as in Survival, but its jumping ability is significantly improved. You cannot out-run a spider in Indev mode. Spiders will not burn during the day, but will also not start to chase you until you get very close.
It has been confirmed that in the future the spider will produce a "web tile". What this will be used for is currently unknown.
A screenshot of the sheep, that appears in version 0.30
Sheep
Sheep are, like pigs, a neutral mob that first appeared in version 0.30. Unlike other mobs, sheep do not stray far from the area they spawn at; they move to an area with grass if they spawn in an area without grass, however. They usually spawn in packs of 2-8. Unlike other mobs, they eat the grass they spawn in. On being hit, they drop between 1 and 3 cloth blocks. If given time, the sheep's wool grows back. On being killed, they do not drop meat. Sheep are often considered completely worthless and not even worth the effort of killing. Although sheep will jump right in front of you and waste valuable durability of pickaxes/axes/shovels if they are given the chance. Though in survival mode, they will drop mushrooms, like pigs.
Future Mobs
Slimes
Not a lot is known about slimes. It has been confirmed by Notch that he wants to implement them, and that they will be able to climb walls.
Cows
As of now, there is no present information, except that they will produce hide blocks. It is currently unknown how they will act. It is possible that some can produce milk, and that they may charge at the player. Since they produce hide, they might be used to craft leather armor.
Chickens
Chickens have been confirmed to be added soon; they will likely drop feathers.