Online poker is not one game, but a large family of related games. Hence the online poker rules may vary from site to site. There are so many variations of these poker rules that it sometimes seems a new "family member" appears every time someone picks up a deck of cards. Still there are general poker rules and principles that unite almost all games that fall under the name of poker.
The object of online poker is deceptively simple: win the money in the pot during a round of play by either having the best poker hand (highest ranking five-card hand) when the wagering is done or by forcing all other players out of the hand. The pot contains all money contributed during a hand, including antes, which are used to seed the pot before each hand begins.
Almost all poker games use a regular deck of 52 cards, comprised of four suits (clubs, diamonds, heart and spades) that each have 13 cards ranking as follows from highest to lowest: ace, king, queen, jack, 10, 9, 8, 7, 6, 5, 4, 3, 2. (Note that the poker rules for most games stipulate that the ace can also be counted as the lowest card).
Each hand of a poker game is, in essence, a self-contained game in which someone will be awarded the pot. A new hand is initiated by the dealer who deals the cards according to the specific poker rules for that game. Each player in turn, proceeding clock-wise from the dealer, may either bet or check (also known as a pass). If a player makes an opening bet, the next player clockwise has three choices: fold, call or raise (Players can no longer check once a bet has been made).
A fold means that the player does not want to spend more money to vie for the pot; therefore, he discards his cards and is no longer a part of the hand. A call means that the player stands in the hand by betting an amount equal to the current bet. A raise means that the player increases the current bet. A reraise is a raise of a previous raise. Betting ends when all who wish to call the highest bet have done so or when all but one player has dropped out. The poker rules for the specific game may limit the number of raises allowed. However if only two players are in the pot they can keep raising each other indefinitely.
The sequence of dealing cards and having a round of betting continues according to the poker rules of the specific game. At the end of the last betting round, all players who are left in the hand have a showdown in which the player with the best hand is awarded the pot. It is the obligation of the player who made the last bet or raise (not call) to show his or her hand first in the showdown.
The other remaining players then show their hands clockwise from the first shown hand. However, the other players do not have to show their hands; if they know they are beaten, they can just concede the pot. Remember that not all hands will reach the showdown stage; very often, a player will win the pot because all other players have folded their cards. These are some basic poker rules that should help you in your game of online poker.
The final bit of information that you'll need to know in order to play online poker competently is the ranking of poker hands. These are covered in the Poker Tips page.