Basically, not all states allow online poker to be played within their borders. However, due to US federal law (The Wire Act) that prohibits moving funds for gambling of any type over state lines, all the players in a poker game must be within the state.
However, it is only legal in 3 states in the US, these being, Michigan, Pennsylvania, and New Jersey. PokerStars also offers a fantastic online poker division for online players, where players can claim fantastic online bonuses while playing exceptional poker games.