It's quite a lot to explain, i'll provide a link to the forum to explain. It's like those pixel car rpg's you guys used to play, you get a job, you work your way up to buy a car, enter comps to win money, build up a sweet car, get it featured, have car shows, meets, etc..
http://www.bnbgaming.net/
EDIT: The forum has since been overhauled quite a bit. There used to be quite a few members on the forum, and it was heavily used until Forza 5 came out. There's a new owner and the forum has been reset but regardless that just makes for a cleaner start.