Agar.io is a massive multiplayer online action game where players control circular cells within a Petri dish-like map. The objective is to grow by consuming smaller cells and randomly generated agar while avoiding larger cells that pose a threat. Players can split their cells and utilize various game modes, including Free-For-All, Battle Royale, and Teams, to enhance their gameplay experience. Customization options allow players to change their cell's appearance using predefined skins and names.

The game introduces strategic elements such as the use of viruses, which can split larger cells into smaller ones, creating opportunities for tactical play. Players can also collaborate by feeding others or strategically hiding under viruses. With its simple mechanics and engaging gameplay, Agar.io provides a unique blend of strategy and action, appealing to a wide audience of gamers seeking competitive fun.