Omaha is similar to Hold 'Em in using a three-card flop on the board, a fourth boardcard, and then a fifth boardcard. Each player is dealt four holecards (instead of two) at the start. In order to make a hand, a player must use precisely two holecards with three boardcards. The betting is the same as in Hold 'Em, using a preflop, flop, turn, and river betting rounds. At the showdown, the entire four-card hand should be shown to receive the pot.
All the rules of Texas Hold 'Em apply to Omaha except the rule on playing the board, which is not possible in Omaha (because you must use two cards from your hand and three cards from the board).