Calves are young Bulls or Cows. Examine them to determine their gender.
Calves are found in the wilderness, you can use a rope on them to lead them, once lead they will not go into water. You can feed them by placing them in a tile of grass, or a tile of farmed field, but they can eat your crops, so be warned.
Calves may wander for food, so fence them in a large area.
Cows, calves, and bulls are weak and rather easy to kill, assuming you have a decent weapon. You should have a weapon of at least QL 15 to kill a bull, but you should be able to kill a cow and a calf with one of the starting pickaxes.