Lowball poker is a way of playing poker in which the lowest hand, rather than the highest hand wins.
There are two ways to rank the hands in lowball poker games. By far the more common variation, known as "ace to five", is to not count straights or flushes as high hands, and simply judge the hands by their rank. Thus, the hand A-2-3-4-5, in any order, would be the best possible hand, even if it is all of the same suit.
The other possibility, known as "deuce to seven" counts straights and flushes as high. In this game, the best possible hand is 2-3-4-5-7.
Lowball can be played with many different forms of poker. The term "lowball" most commonly refers to draw poker, and deuce to seven is usually only played as a draw game. Lowball draw is often played as "triple draw", in which rather than one draw and then a showdown, there are 3 draws, each followed by a betting round, followed by a showdown. Another common variation is known as razz, which is seven card stud played as a lowball game.
Some games use a split pot format: that is, the highest hand wins half of the pot, and the lowest hand wins half. This is a common format in Omaha and Stud games. Some games allow any low hand, while some require that the low hand be composed of 5 unpaired cards below 8. These games are usually called "8 or better", as in "Omaha Hi-Lo 8 or Better", or simply "Omaha 8 or Better". Hi-Lo games almost always use the ace-to-five format.