Cow: A cow is a female animal that has had at least one calf.
Heifer: A heifer is a female animal that has never had a calf. Once a heifer has a calf, she automatically becomes a cow.
Bull: A mature male animal that is used for breeding. (And for the record, I did know what a bull was when I initially asked my city kid question!)
Steer: A steer is a male animal that has been neutered.