Cow

Cows are passive mobs. They are the primary source of leather, raw beef, and milk. They are 1.6875 blocks tall, 0.625 blocks wide, and 1.5 blocks long.

Appearance
Cows have black/brown skin with white spots. All cows have a pair of horns on their head. They are most likely modeled after the Norwegian Red Cattle.

Behavior
Cows roam in the Overworld, usually in herds of 4 to 10 (however it is not uncommon to find a lone cow). They tend to walk around slowly and aimlessly, mooing, but they will flee and jump wildly if attacked. They also tend to jump off high cliffs (and die in the process). Cows make no attempt to stay out of water and will follow the player when he/she is holding wheat, which is used for breeding. Cows will make even deadly attempts to follow the player. They also can climb ladders in certain conditions. A cow can be milked by right clicking the cow with an empty bucket. A cow may be milked an unlimited number of times.

Xbox 360 Edition
They are unaware of cliffs and holes in the ground, leading to them occasionally falling and dying. They drop leather and raw beef on death (as of 1.8) and can be milked. They are usually found in herds of around 4 cows, but can sometimes be found alone. If they die while on fire, they drop steak instead of raw beef.

Breeding
Breeding cows with wheat will create a calf and may give experience. The parent cows have a delay of 5 minutes until they can breed again. Calves will follow their parents or other adult cows until they're fully grown. A calf looks similar to an adult cow, but its body is smaller while the head remains the normal size of a full adult cow.

Calves move faster and take 20 minutes to fully grow. They do not drop anything when killed. However, they can be milked.

Farming
Trapping a herd of cows in an enclosed area and breeding them is beneficial to the player. Cows drop more types of useful resources than all other passive mobs (excluding the rare Mooshroom), thus providing the player with an unlimited supply of steak, milk and leather.

Breeding two cows also has a chance to grant a small amount of experience.

Bugs
Cows will sometimes spawn trapped under jungle leaves or in blocks. While the cow is following a player, it may glitch and start walking back and forth or side to side in attempt to follow the player. To fix this, punch it. This will temporarily stop the cow from following you.

Trivia

 * Cows in the PC Gamer Demo of Minecraft are branded with a 'PCG' logo on their side.
 * Because of the lack of hierarchy in every box model, the horns of the cow are sometimes misplaced. Jeb is planning to fix this.
 * Notch stated in a tweet that all breedable peaceful mobs displayed traits of both genders and could breed with any other individual of their species. This also applies to cows, since they have horns and give milk (although some cow females of certain breeds do have horns).
 * Cows are able to climb up elevation but are unable to climb down inclines lower than two blocks.
 * Because cooked beef is equal to cooked porkchops, it is more profitable to breed cows instead of pigs. While pigs only provide porkchops; cows not only provide beef, but also milk and leather. Also cows need wheat to breed, which is easier than carrots to obtain, which are needed to breed pigs.