Spawning
Cows will spawn naturally in the Overworld on Grass Blocks. They can naturally spawn in any biome other than dessert and ocean biomes. As well as this, cows can be spawned from a Cow Spawn Egg which you can get from the creative menu.
Drops
Drops are items which are 'DROPPED' by an animal. This usually happens when the animal is killed, unless specified.
Breeding
Two adult Cows can breed to make one baby cow by giving a piece of wheat to each of the adult cows.