Agar.io is the one game that served as the catalyst for the emergence of IO games. In it, you consume anything that is smaller than you, including other players, as you eat, grow, and then eat some more. It resembles Osmos, a game that is somewhat similar, but Agar.io is multiplayer and free. You entered a room with a large number of other players, all of whom were vying to grow, eat, and consume one another in order to become the largest blob they could. If you passed away, you had to restart. The game gained enormous popularity, and it has since given rise to hundreds of other IO games, some of which are very similar to Agar.io and others which are not, but which all share a common structure. They are all multiplayer and completely free.
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Although a few io games deviate from this pattern, most IO games adhere to a similar set of rules. In general, expect an IO game to be:
Playing is done quickly; you can jump at the touch of a button and begin playing right away. All of them are browser-based and only require small downloads. They are a significant improvement over older flash games in many ways.
The majority of IO games are simple to pick up after just a few seconds of playtime or a few seconds of reading a tutorial. But it can take some time to become proficient with them (and countless deaths from players who are exponentially more powerful than you).