Four cards in hand, but exactly two of them must play.
Omaha deals four private cards instead of two, and shares the same five community cards as Texas hold'em — flop, turn and river, with a betting round before the flop and after each stage.
This is the rule that catches everyone. You must use exactly two of your four private cards and exactly three of the community cards. Not one, not three. If four hearts are showing on the table and you hold one more, you do not have a flush — you need two hearts in your hand.
Because every player has six possible two-card combinations to draw from, hands run much stronger than in hold'em. A flush that would win a hold'em pot is routinely second best here.
Omaha is usually played pot limit rather than no limit: the largest bet allowed is the current size of the pot. A raise is calculated by first calling, then raising by the pot as it stands after that call.