5 Card Draw Poker is played by 3 - 7 players. Each is dealt five cards and there is a round of betting based on the hands. After the betting players can now put up to three cards back in exchange for new ones. (There's one exception where a player can exchange four cards if he shows the fifth to be an ace).
There's a further round of betting and the best hand wins. Draw poker was once the standard way to play but it has of course been overtaken by Texas Holdem.