The best cards that determine what the poker hand is. Or the cards in ones possession that work with community cards. Such as saying my starting hand was AK in Texas Hold'em.
A list of poker hands at Wikipedia.
A conventional poker hand is five cards. Many variations of poker and games that use poker hands to determine out come, deal the player more or less then five cards. The number of cards, how the ranking of cards is determined is not standard and often what constitutes a hand is the variation that makes a particular poker game a variant of another poker game. It should be noted that poker is not a card game but a betting game. The scope of what makes a poker hand is very wide. Liars poker is just one example of playing poker with out cards.
Texas Hold'em is a game were seven cards are used. The best hand is the best five card poker hand. See A short discussion on kickers at Wikipedia. The players best five cards can be any five cards, even if the best five cards are all in the community cards.
In Omaha Hold'em each player has four cards in their hand with five community cards on the board. To make an Omaha hand the players best five cards must consist of three community cards and two cards from the players hand. In the hi-lo version the rule is the same, however the two cards for the high hand do not need to be, but can be the same two cards that are used for the low hand. More at Wikipedia.
See Community Card Poker at Wikipedia. For more on what makes a hand on particular variations of community card poker games.
In Seven Card Stud each player has seven cards where they use the best five cards to make up their hand. More at Wikipedia.
Other poker like variants, three card poker and Chinese poker, have parts of the hand that may consist of less then five card hands.
Often a point of confusion is how suits may determine the ranking of a hand. Generally speaking Suits are not considered when determining rank of a hand at showdown. Meaning If two players have an identical flush, no suit determines which flush is higher. Identical hands where two or more players have the same hand, the players are tied and split the pot equally. The only time suits count is in games, like Stud when where the action starts is determined by the highest or lowest up cards and sometimes with a chip race in tournaments. In this case suits are ranked highest to lowest alphabetically, Spades are highest, Hearts, Diamonds and Clubs lowest.