Online poker is the game of poker played over the Internet (online). It has been responsible for a dramatic increase in the number of poker players worldwide, and as of December 2003, revenues from online poker were estimated at US$34 million per month.You need a multiplayer poker software to play poker online.

Traditional (or "brick and mortar", B&M) venues for playing poker, such as casinos and poker rooms, may be intimidating for novice players and are located in geographically disparate locations. Brick and mortar casinos are also reticent to promote poker because it is very difficult for them to profit from the activity. Though the rake, or time charge, of traditional casinos is often very high, the opportunity costs of running a poker room are even higher. Brick and mortar casinos often make much more money by removing poker rooms and adding more slot machines.

Internet poker, on the other hand, are dramatically cheaper because they have much smaller overhead costs. For example, adding one more player table does no require more physical space. Internet poker rooms tend to be viewed as more easier to play. For instance, the multiplayer poker software may remind the player when it is his or her turn to act.

Online poker rooms also allow the players to play for very low stakes (as low as 1¢) and often offer poker freerolls (where there is no entry fee), attracting beginners.

Internet may be more vulnerable to certain types of fraud, especially collusion between players. However, they also have collusion detection abilities that do not exist in brick and mortar casinos. For example, online poker room security employees can look at the "hand history" of the cards previously played by any player on the site, making patterns of behavior easier to detect than in a casino where colluding players can simply fold their hands without anyone ever knowing the strength of their holding. Online poker rooms also check player's IP addresses in order to prevent players at the same household or at known open proxy servers from playing on the same tables.

The major online poker sites offer varying features to entice new players. They also run best multiplayer poker software. One common feature is to offer tournaments called satellites by which the winners gain entry to real-life poker tournaments. It was through one such tournament that Chris Moneymaker won his entry to the 2003 World Series of Poker. He went on to win the main event causing shock in the poker world. The 2004 World Series featured triple the number of players over the 2003 turnout.