It breaks down like this:
In the State of Delaware, there are state lottery sites on which you can play online poker, using the 888 platform. You only play with players within DE and NV. They also have casino games for players in DE only.
In the State of Nevada, you can play online poker (only). The 888 powered site(s) there share players with DE. The other site (Real Gaming) does not share players.
In New Jersey, you can play online poker on 888 and Party Poker platforms, and in early 2016, a Pokerstars platform. NJ sites are NJ players only. NJ also has casino games for players within the State of New Jersey.
No other states currently have regulated online gaming. Some are considering it (CA, PA, WA, NY, etc) but all are a long way off.
Unregulated sites (Bovada, ACR, BCP, Carbon, etc) still serve most states in the US. The legality of that service is subject to debate.