Diep.io is a free online multiplayer game where players control tanks in a 2D arena. The objective is to destroy objects and other players to earn experience points, which can be used to upgrade the tank's abilities. Players can choose different tank classes and customize their playstyle as they progress. The game features various modes, including team battles and free-for-all, allowing for diverse gameplay experiences. Players can also compete against AI-controlled shapes to practice their skills. With simple graphics and easy controls, Diep.io is accessible to players of all ages.