Grand Theft Auto Online is the online portion of Grand Theft Auto 5, and has continued to be one of the most populated online games since its release in 2013.
In Grand Theft Auto Online is separate from the base game. Players can hop into a session of up to 30 people with their own custom character to play around in the city of Los Santos without the constraints of Grand Theft Auto 5's story. From there, they can drive, gamble, fight, and participate in missions with other players.
Despite its age, Grand Theft Auto is booming in players and likely will not be usurped until Rockstar releases Grand Theft Auto 6 and its probable online counterpart.