Cows are passive mobs found in the overworld, like the pig.
If you have a bucket on hand, you are able to milk the cow by right clicking it. When you kill a cow you can receive raw beef and/or leather. If you kill the cow by fire you will receive steak, instead of raw beef. By giving two adult cows wheat, they will reproduce and a calf will appear. This calf will take 20 minutes to fully mature.