Limit hold'em: bet sizes are fixed, with a cap on raises per round.; Pot-limit: the maximum bet is the current size of the pot.; Omaha: four hole cards, and you must use exactly two of them.
Why people argue about this
People often assume that in Limit Hold'em, you can only bet as much as your opponent did before, which is incorrect. The actual rule allows for fixed bet sizes per round with a limit on how much you can raise after seeing the flop or subsequent rounds. Confusion arises because they misinterpret the "fixed" part of "Limit.
