basically you start the game with a foal. Virtual, of course. You have to take care of it each day, play with it, then put it in competitions etc., then after a while you start your own equestrian centre, and breeding farm, etc. etc. So far I have 29 days' membership and 6 horses! Not very much money but I'll get more soon enough
It's good fun - when you first play the game there's not much to do, but as you get more horses, and a centre etc. it becomes a lot more fun.