Gaming etiquette (also called gamer etiquette or video game etiquette) refers to the norms adopted while playing multi-player video games. While specific genres and games have their own accepted rules of conduct, some of these rules are universal across almost all games.
Universal gaming etiquette
Regardless of the game, certain behaviours are universally encouraged or discouraged in multi-player video games. Cheating is almost never acceptable unless all players agree it should be allowed, as it causes the game to become unfair and detracts from the enjoyment of legitimate players. In games involving communication between players, it is considered impolite to brag and gloat about one's accomplishments, though this is more acceptable in some games than others. In games where most communication between players is through text, it's usually considered rude to type in all caps (which is known as shouting). Camping in games such as first-person shooters is generally a legitimate strategy, though some players consider it to be unfair or bad manners. In games with split screen(played on the same screen) it is considered unsporting to look at an opponents actions by looking at their portion of the screen. This method is also known as screen watching, screening, screen cheating, screen peeking, screen hacking, or screen looking.
In any game, players are generally encouraged to be polite and courteous to one another, and avoid excessive profanity and trash talk. Players are also expected to be friendly and welcoming to newcomers, and remember that their lack of skill or understanding of the game is only due to their lack of experience with the game. It's also a generally accepted rule that a game's events and outcomes shouldn't be taken personally by the players.
Variations between games
In most games, it's considered bad manners to quit before the game has ended, especially in games where a player's disconnect will trigger a spike in latency which causes the other players to have to wait. However, in other games such as Starcraft II, the opposite is true: it's considered bad manners to stay in a game after a clear victor has been decided because the victory requirement to destroy all of an opponent's forces is seen as a waste of the player's time. However, in games where leaving early is encouraged, the player is often still expected to say 'gg' or 'good game' before leaving; failure to do so can be considered bad manners. However, the winning player saying 'gg' on behalf of a losing opponent is impolite, as it suggests the game has already been won and the other player should quit (this is known as an "offensive gg"). 
In games involving circumstances where many players need to meet at an agreed-upon time, such as in a raid in World of Warcraft, it's considered bad etiquette to show up late or leave the computer during a game (AFK) without the consent of teammates.
In team games where the teams are automatically balanced, it's impolite for players to stop playing rather than simply leaving the game and allowing someone else to take their spot, because this can put their team at a disadvantage.