Betting Round: If any cards of players' hands are up, betting usually begins with whoever is showing the highest hand. Some people play that in high/low games, the first round of betting starts with the lowest showing hand and all later rounds start with the highest. If no cards are showing, the betting should start with a different player each round, beginning with the player to the left of the dealer and rotating to the next player clockwise each round. A round consists of a series of bets until everyone has called or folded.
Declare: Announcing whether you're trying for the high hand, low hand, or both. Typically, this is done by secretly putting no chips in your hand if you're going low, one chip if you're going high, or two chips if you're going both. Everyone then opens their fists simultaneously.
Down: Face down, so that the value of the card isn't showing.
Exchange: Discard cards from a hand and replace them with different cards, usually from the top of the deck.
High/Low: The highest hand and the lowest hand split the pot. In many high/low games, it is possible to win the entire pot by having the highest and the lowest hand (accomplished via wild cards, or by using different sets of 5 cards), but only if you declare that you're trying for both. If you try for both but don't win both (that is, someone else has the highest or the lowest hand), you win neither. When playing high/low, hands are only compared against those declared the same way unless you're playing that cards speak for themselves, in which case there's no declare and all hands are compared against each other. If someone goes for both and fails, say by winning high but losing low, the high half of the pot goes to the low winner-- it doesn't go the second best high hand.
Natural: Without wild cards.
Perfect low: A perfect low is usually A-2-3-4-6. However, it can also be A-2-3-4-5 if you play that straights don't count when you go low, or it can be 2-3-4-5-7 if you play that Aces are always high, or 2-3-4-5-6 if you play both of the above.
Rank: Value of a card. There are 13 ranks in a deck of cards.
Roll: Flip a down card up.
Rolling Showdown: A showdown in which cards are stacked and revealed one at a time with a round of betting after each card is revealed.
Rotating Bet: Each round of betting starts with the next player clockwise. Typically, the player to the left of the dealer starts the first betting round. The 2nd round would thus be started by the player to his left, and so forth.
Showdown: The process of revealing hands and determining the winner.
Trick: A set of cards, one card from each player, played one at a time in sequence around the table. Tricks are usually found in games like Hearts and Bridge, not in true Poker games.
Trump: One suit which is stronger than other suits. The lowest ranking card of a trump suit beats the highest ranking card of any other suit. Trump suits are used in trick-taking games and are not found in true Poker games.
Up: Face up, so that the value of the card is showing.
High Card: 2 is lowest, Ace is highest
One Pair: Two cards of the same rank
Two Pairs: Two cards of one rank and two cards of another rank
Three of a Kind: Three cards of the same rank
Straight: Five cards in sequence. Aces can usually be high (10-J-Q-K-A) or low (A-2-3-4-5).
Flush: Five cards of the same suit.
Full House: A pair and three of a kind.
Four of a Kind: Four cards of the same rank.
Straight Flush: Five cards in sequence which are also of the same suit.
Royal Flush: 10-J-Q-K-A of the same suit.
Five of a Kind: Five cards of the same rank. This is only possible with wild cards.