What poker is and isn't
Poker is not one game but a variety of games that employ hand rankings, betting, and bluffing as strategic and tactical elements. In some forms of poker, like SevenCard Stud, Texas Hold'em, Five-Card Draw poker, and Omaha, the best poker hand wins. What's the best hand? The rarer the hand, the higher it's ranked. Thus a straight flush, which is much less likely to occur than a full house, is ranked higher. That's why three-of-a-kind beats two pair, which in turn beats one pair.
In other games, like Lowball, Razz, or Deuce-to-Seven Kansas City Lowball, the lowest-ranked hand wins. In all but Kansas City Lowball, a hand composed of 543-2-A - called a wheel or a bicycle - is the best possible hand. In Kansas City, where straights and flushes count against you and an ace is always a high card, the best hand is 7-543-2. But don't worry about that game, it's not played very often. When it is, it's generally a high stakes, no-limit game - one you'd be better off avoiding for a long, long time.
If this isn't enough to confuse you, there are also split games, in which the highest and lowest hand split the pot. These games are usually played with an "8 qualifier." In order to split the pot, the low hand needs to be composed of five unpaired cards with the rank of 8 or lower. If there is no low hand, the high hand scoops the entire pot.
In casinos the two most popular "split" games are Omaha 8-or-Better, High/Low Split (usually abbreviated as Omaha/8) and Seven-Card Stud, 8-or - Better, High-Low Split (usually abbreviated Seven-Stud/8).