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 effected by the same environmental hazards as the player, including drowning, burning and fall damage.
[Unconfirmed] If any of these mobs are trapped in a pit or kennel and left alone for too long, they will de-spawn
A human
Humans
Humans are non-player characters (NPCs) which take the form of clones of the default skin, created by pressing 'G' while playing in Creative mode. If the player has a custom skin, any humans created still have the default skin and do 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. Humans currently do not appear in Survival or indev. The original human model was originally going to be used for a Grand Theft Auto: Chinatown Wars clone by Notch. Currently the ability to spawn these mods has been disabled by Notch purportedly to prepare for future usage.
A pig, aimlessly wandering
Pig
The pig is a neutral mob that appears in survival 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 not hostile, pigs can bump around the player, and possibly push the player off a cliff to the player's untimely death. As of now, Pigs drop anywhere from zero to two units of pork when they are vanquished.
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 take 4 to 6 hits to kill, making them one of the more durable enemies, and inflict 2 hearts of damage upon attack. Skeletons sometimes wear chest armor or helmets in survival, but do not yet do so in indev. As of the update of September 3, 2009, skeletons can shoot arrows, something that can be exploited by the player to cause in-fighting. Killing skeletons results in arrows being fired radially from the corpse onto the nearby ground, to be collected by the player. When a skeleton is killed in survival, the player gets 120 points. In indev, on being touched by sunlight, skeletons burst into flames.
In <Free> Survival mode "Boss" Skeletons appear randomly along with groups of normal skeletons. Boss skeletons are easily distinguishable by their protruding shoulderpads. These skeletons are very dangerous, they shoot Three(?) Arrows at a time, and coupled with the power of the skeletons around it, will make short work of you.
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 skeletons upon being touched by sunlight in indev, they are ignited.
In <Free> Survival mode "Boss" Zombies may spawn along with a small group of normal zombies. Easily distinguishable by it's protruding shoulderpads it should be attacked with caution. The boss zombie is harder to kill, and [Unconfirmed] may deal more damage.
A Creeper, almost dead
Creepers
The Creeper is a type of mob, found in Survival and indev. In indev mode they only spawn in dark areas, such as unlit areas on the surface at night, or in unlighted caves. While its attack only does 2 hearts damage instead of 3, it behaves the same as a zombie. After taking some damage, it flashes, showing it is ready to detonate. Upon being killed, it explodes, which can damage the player and creates a small crater, destroying plenty of blocks. Unlike the zombie, it cannot obtain armor. In groups, they can cause a chain reaction of explosions, which can be very dangerous to the player. Originally, creeper explosions destroyed all tiles in their vicinity. Currently, their explosions have no effect on rock, cobblestone, ore, or metal tiles in Survival, and a minimal effect in indev.
Their explosion behavior is also different in indev. Creepers don't physically attack you at all, instead they just try to get close to you and then they self destruct. This is very problematic if you are hiding in a base during night time, since the resulting hole could well allow much more dangerous mobs in. They can also see through glass, and do not catch fire when daylight comes. As such, be careful how close you get to creepers when trying to exit your base or when going into an exposed part or even a greenhouse. Also unlike survival is that if you manage to kill a creeper before it gets too close, it won't detonate upon death. When a creeper is killed in survival, the player gets 200 points. The Creeper design is based on an attempt of a pig model, hence the four legs.
A spider
Spiders
Spiders behave like zombies, but jump at the player faster, making them harder to hit. While arrows make them easy to deal with, in packs they are extremely dangerous. At close range, their jump makes you an easy target. In indev mode they only spawn in dark areas, such as unlit areas on the surface at night, or in unlighted caves. During the day, spiders will not attack unless provoked. It has been confirmed that they will eventually produce web tiles. What they will be used for is unknown. As the second mob in the game, the spider does not have the same length, width or height as other creatures; instead of 1×1×2, the spider is 2×2×1, which means it can run under a 1-block-high area. When a spider is killed the player gets 105 points. In indev, their eyes glow. The spider is the only mob with multiple skins.
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 a couple of cloth blocks. If given time, the sheep's wool grows back. On being killed, they drop meat which refills a portion of the player's health when consumed.
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.