Cows are passive mobs that were first introduced in the Alpha 1.0.8 update.
Cows are capable of breeding like every passive mob, except for Squids. You simply put them in a fence farm with one fence gate and feed them wheat.
When you hit over 2 cows, you should kill them for leather, then stop when you hit 2 cows. This is a quick way to gain leather, for armor. Although if you hit large veins of iron, then use that for armor instead.