The answer to this is quite simple (at least to anyone who has spent time on a farm). A heifer is an adult female bovine that has not yet given birth to a calf. Usually a heifer is about one year old, at which point they are bred with a bull (not a steer, which has been castrated, or neutered). A heifer becomes a cow once she has given birth. A calf, of course, is a juvenile bovine, male or female. Sometime usually after their first birthday, a male calf is either castrated (at which point it becomes a steer, usually used for beef) or allowed to develop for breeding (as a bull). Cows are used for breeding but are also of course used on dairy farms. To find information on how cattle are raised, you should take a look at a university agricultural extension department. These exist to provide information for farmers and others. I have linked to a couple of these sites below.