Massively multiplayer online games, or MMOs, with an emphasis on reflex- and accuracy-based challenges are MMO shooters. Such games usually give each player control over an avatar bearing a weapon of some form, usually a gun. The player controls his avatar and attempts to complete a variety of different challenges or objectives that may involve cooperating with or competing against other avatars controlled by other players. MMO shooters can very significantly by genre, scale, game objectives, aesthetic style, and a range of other factors.
There are many different settings used for MMO shooters. Some such games are in science fiction settings while others are based in historical settings such as World War II. The setting of the game drastically impacts the art direction, appearance of avatars and weapons, gameplay dynamics, stories, and many other important aspects of the games.
The implementation of multiplayer gameplay in MMO shooters is another important defining factor of such games. In most cases, players join teams that compete against other teams, either in an online world or in timed matches. The statistics of each player and team are tracked over time, and the objective is to have the best personal and team score. In a few games, there are no set teams and matches set all players against each other, so only a player's personal score is important. Other MMO shooters are primarily cooperative, prompting players to join together against computer-generated opponents and challenges.
Few massively multiplayer online games are purely shooters. Many, for instance, incorporate role-playing game elements such as character building and combat based on quantitative skills assigned to the avatar and developed through gameplay. A player who performs well may get to improve some aspect of his avatar, such as speed or aiming ability. Other MMO shooters are purely shooters, which usually means that they are mostly, if not entirely, based on the skill of the player rather than on the "skill level" or special abilities of the avatar.
Another major difference between different MMO shooters involves the expense to the player. Some MMO shooters are completely free to play and are usually supported by advertisements. Others require players to purchase the game first or to pay a monthly fee to keep playing. Still others offer the basic game for free but charge money for extras such as improved weapons or unique character models. This payment model is particularly popular as more people are likely to play the game and eventually want the expanded features if they can initially play for free.