The river is the fifth and final community card to be dealt. It is of course also used in combination with the other cards to form each player's hand. In the fourth round of betting you can bet once more. After this round of betting, the poker players who are still in the hand show their poker cards. This is called the showdown.
At the showdown all the players still in the hand reveal their cards, to find out who has the best holdings and wins the pot. Each poker player's hand is the best five-card hand he can make from exactly three of the five cards on the board, plus exactly two of his own cards.
In Limit Omaha, there are predetermined bets for each round. For example in a $10-$20 Limit Omaha game, all bets and raises during the betting rounds before and immediately after the flop are $10, while bets for the turn and the river are $20. Typically the big blind in Internet Limit Omaha is equal to the bets required in the first two rounds, and the little blind is half a bet. In addition, all bets and raises must be greater than or equal to the previous bet or raise, with the exception of a player that is raising all in. If a poker player does raise all in, and that bet is not greater than or equal to the previous raise, then the door is not opened for the original raiser to reraise.
Pot Limit Omaha rules are the same as fixed Limit Omaha rules, with the exception that the maximum bet is equal to the current pot size. A player may bet or raise any amount they wish, up to the amount that is currently in the pot. There are still minimum bets. If you are playing 10-20 no limit Omaha Hi/Lo, for example, then you have to bet at least 20 dollars in the first two rounds, and at least 20 dollars in the latter two rounds. In addition, all bets and raises must be greater than or equal to the previous bet or raise, with the exception of a poker player that is raising all in. If a player does raise all in, and that bet is not greater than or equal to the previous raise, then the door is not opened for the original raiser to reraise.