Yeah there is a cap. It's really not very chaotic.
Basically you create a character then try to make money. With money you can buy clothes, mod cars, buy houses, property and weapons, just like in single player.
You make money by doing missions (guys like Lester, Trevor, etc will hit you up for missions you can do by yourself or with your friends). If you're part of a crew, you can at any time play missions, races, jobs, etc and make money).
There is also free roam where you can run around and do whatever like rob liquor stores, 711, evade the police, etc.