A faction is a group of players (sometimes called a clan or guild) that band together for a common purpose. A faction can have one or many players and it’s members can claim land and hold territory. This claimed land is protected from non-members of your Faction by preventing other players from placing or destroying blocks within the boundaries of this territory.
Factions participate in many types of game play like playing defence against other teams to protect items and their base against other factions, pvping for items, and simply being social. Because faction members have access to the faction base and items, other players may try to join your faction for the sole purpose of raiding your faction. So be careful when adding new members to your faction. Remember, a group of good friends are always the best choice for a faction.