In the United States, online casinos are only legal in certain areas. Slots can be played with wagers of real money at these online casinos, but the player must be physically present in the legal area when playing.
As of May 2015, online casinos are legal and available in Nevada, Delaware and New Jersey. The IP address of players is usually checked to verify it is within the legal area, or a player's phone may be periodically checked for its physical location.
Virgin, Caesars and Golden Nugget all have virtual casinos with slots in New Jersey. The Dover Down casino in Delaware also offers online slots.
Nevada only offers online poker at present, with traditional brick-and-mortar casinos using their clout in the state to resist online slots. There is some gray area with Facebook slot games run by major casino properties such as MyVegas (run by MGM) and Ceasars Casino and Slotomania (run by Caesars). These games offer slots that are free to play, but players can opt to purchase virtual tokens with real money and may need to, as only small amounts of free tokens are doled out daily. These games are legal throughout the United States because real money is not being directly wagered.