Online gambling is a major legal gray area in the United States. Several states do have wording on the books that would seem to indicate it is outlawed in those states. (North Dakota, for example, has charged and fined an online gambler.)
The U.S. Department of Justice has been pursuing media outlets that advertise online gambling. No gamblers have been arrested or cited yet by the feds, although a handful of casino and sports book operators who reside in the U.S. or who are American citizens overseas have been arrested and convicted of violating the Wire Act, the only federal law that has been applied to online gambling.
The wording of the Wire Act is interpreted by most legal professionals as being more specifically directed toward sports and horse-race betting. Anti-gambling advocates like to take a broader interpretation and apply it to all online gambling. The Act itself reads "... placing of bets or wagers on any sporting event or contest ..." and that's part of the confusion.
Another ambiguity is the wording of the Wire Act where it reads "uses a wire communication facility" for bets. The law was originally intended for telephone lines, but some broader interpretations consider the Internet as falling in this category.
If you have any concerns about whether online gambling is legal, you can choose not to play. You should also consider researching your own state's laws (if any) regarding this.
We list a couple of poker rooms on our online gambling site at www.LVAGO.com. If you click on the links thre, you can download the software from them. Each of them has a play-money mode where you can practice without putting any money at risk. If you'd like to play, you can bet as low as $.01/$.02 and as high as $80/$160. They also have tournaments that are fun to play. It's highly recommended that you use the play-money games, as they're excellent initial learning resources.