Agar.io is the first popular IO game. Nowadays its classic, or even retro. In agario-like IO games you play 2D style multiplayer online game with some sort of creature that eats some sort of food and grows bigger in consequence. In some games bigger players can eat those who are smaller. In other games you can split your creature into several smaller creatures and try to beat your opponent in that way. The bigger you are — higher scores you get. Top players are usually listed at leaderboard.