Community Cards are cards dealt in a poker game face up into the center of the table. They are available for any or all players to use, and are considered to be simultaneously part of every player's hand. Any or all players may use whichever of the community cards that they desire in making a poker hand, subject to the specific rules of the form of poker they are playing.
Some forms of poker (such as Hold 'em and Omaha) use community cards while others (such as Seven-card stud and Razz) do not, but those that do have become increasingly popular in recent years.