Omaha

Omaha is similar to holdem in that you have to make your best 5 card hand, but you make this from 9 possible cards(5 of which are shared like holdem).

Rules
Same as Hold'Em

except:
a) each player is dealt 4 pocket cards(instead of 2). there are still 5 community cards.
b) in order to make a hand, the player must combine exactly 2 of his 4 pocket cards with 3 of the 5 board cards.

Omaha Hi-Lo
Sometime omaha is played hi-lo, which means the pot is split between the highest and lowest hand. The same player is permitted to have both the high and low hand. A low hand must be 5 cards of 8 or less, with no pair. The lowest hand is A,2,3,4,5. There may be no low hand.