Although poker in various forms has been around for hundreds of years, none of the riverboat gamblers or the owners of casinos could have predicted how the internet would change the gambling landscape. Casinos had spent a lot of money to prevent the expansion of gambling to keep the gamblers coming to their establishments, especially in Las Vegas and Atlantic City.
Then a number of Indian tribes entered the market having established their right to ignore state regulations, and casinos found themselves with a lot of competition, since the customer appeared to want gaming more convenient to their home. Some then realized the opportunity that the internet provided to provide gaming with no travel at all. Additionally these casinos had no 'brick and mortar' investment to be made.
In 1998 the first online casino opened it cyber doors (Planet Poker). Sometimes begin second is better than being first, especially with something based on new technology. Paradise Poker open up shortly after, but had the advantage of learning from the mistakes and oversights of Plant Poker, and they quickly became the most popular site. Now there are numerous sites for free and money games. If you have seen any professional poker tournament on TV you have seen the shirts, hats, and jackets with their names emblazoned on them (Party Poker, Full Tilt Poker, Poker Stars, etc).
Almost every winner of the World Series of Poker in the recent past has begun their career playing and learning online. The benefit of low-cost and lots of opportunity to play hands during a day, help the player to practice and evaluate their strategies. Chris Moneymaker, a winner of over $8 million in the World Series of Poker, began online and actually won his entry fee playing in an online tournament. Clearly he was able to develop his skills enough to win in his first live tournament. When that happened it encouraged a whole new crop of young players to pursue becoming a professional poker player.