I think the best way to think of this is it's kind of like a pseudo-MMO. You still have to connect to various games or start your own free mode like you did in IV, but when you're in you can do pretty much every activity that's in the SP like golf, tennis, etc (in IV the few activities it did have were locked out of multiplayer) and you also have your stats saved to the server (clothes, properties, etc). There's also supposedly a ridiculous amount of missions/jobs to do, which many seem to have their own cutscenes and stories like a regular single player GTA would.
Not to mention other stuff like an editor for making your own custom races and I think even custom jobs.