What's the Story of a Cow?

A lot of people have the idea that there is a cow on a farm somewhere and maybe it's kept in a small cage or something, which is pretty bad. So if I go and get a glass of milk or something, what's the story of that milk? Where does that come from? People do it because their animals have been slaughtered, but they don't realize that dairy cows are still slaughtered as well.