its a legitimate business and there are some legitimate sites. id recommend looking around, making sure you find a site that is popular and you can trust, there should be some sort of verification thing you can check. and try to find out if anybody has had any problems with it before.
the thing i would be worried about is if the cards are truly random, or are they trying to set up outcomes. also if you play something like poker, you could be playing against a couple people who are talking on the phone and teaming up to get an advantage. so i wouldnt really trust that. blackjack it doesnt really matter if people talk, as its everyone against the dealer, but still, i dont know if id trust their "random" cards.
i did do some sports gambling a while back. (actually, it was interference that got me into it. someone posted a link to a place where you could wager on the first song u2 played during halftime of the superbowl, and then someone else posted they had heard the rehearsal and knew the first song would be beautiful day, so i thought, easy money, and it was...) i trust sports gambling more because the sites cant rig the games or anything, so the only thing you have to worry about is if they really pay out, and that would be easy to check, no one would bet there if people knew they wouldnt get paid. and thanks to a nice sports wager i made a hefty sum, had it wired to my bank account and have retired from online gambling with a nice profit (plus its so nervewrecking i couldnt take it anymore).
so that is my advice, but most of all, if you get into it, dont be an idiot. be sure to wager only what you can afford to lose, and if you lose it, dont go back. it can become addicting, so set a limit and be smart and you should be alright.