"Clan Wars" may refer to a massive multi-player battle in the video game "Clash of Clans," a game designed for mobile devices. It may also refer to a mode in "Call of Duty: Advanced Warfare," says the Activision support page, but is more commonly associated with "Clash of Clans."
"Clash of Clans" falls under the category of MMO, a massive multi-player online game, says Supercell.com, the maker of the game. It is a real-time strategy game in which players create a character and attack other players, build villages and train armies. In single-player mode, characters attack strongholds populated by non-player characters such as goblins, giants and dragons. The goal is to accumulate treasure and elixirs to make villages bigger and more well-defended, and to make armies stronger.
Clan wars occur when groups of players team up to fight other groups of players. The battles occur on unique maps, so the home villages and treasure of individual players are not threatened, lessening the consequences of losing the war.
Two days are required for a campaign. The first day is used for preparation, and the second for the battle itself. Each player is allowed to attack twice on battle day, earning stars for victories. The clan with the most stars at the end of the battle is the winner of the war.