Pot Limit Omaha (or PLO) is one of the most action-packed and popular games to be found online. It's a great balance of skill and luck, with many more big hand showdowns, and big pot action.
Pot Limit Omaha plays exactly as per Texas Hold'em, but with players being dealt four hole cards instead of two. While Hold'em allows you to use one - or even none - of your hole cards in your final five-card hand, PLO requires that you use exactly two of your hole cards in conjunction with three of the community cards at showdown. For example, holding the ace of clubs in your hand when there are four clubs on the board would give you a flush in Hold'em, but in PLO you would need two clubs in your hand in order to have a flush in this scenario.
As with Texas Hold'em, PLO uses the system of forced bets called 'blinds'. These are two mandatory bets made before the cards are dealt. The small blind is always posted by the player on the immediate left of the dealer, with the big blind being to the immediate left of the small blind. These blinds slowly rotate around the table with each new hand to ensure all players pays them equally.
Although it can be played as both a limit or a no-limit game, Omaha is nearly always played as pot-limit game, with betting up to and including the size of the pot at any point, but no more.
Yes, Omaha can also be played as a High/Low split game. Find a full guide to this form of Omaha here.