Basically, it's a small city. You can *deep breath* do missions, join a triad, have gang wars, rob other players/shops/banks/triads, buy an apartment then furnish it, get in trouble with cops, buy augmentations/weapons/stuff and get more powerful, explore the city (main city (in 3 chunks), slums, abandoned slums and China town) and so on. The map is geographically huge and the mod brings a lot of new features to DXMP. In all, it's a new experience and pretty fun.
When you break the law, expect to deal with the cops. They'll spawn within a certain distance of you in certain populated areas when you're wanted. If you're captured, you're imprisoned for a time proportional to your crimes (which you can then wait out in jail or get someone to break you out).
Hear a sound of a cocked pistol then you know one of these guys are on to you.