IO games is a category of free online multiplayer games that a united by the following characteristics:
they a quick to play; they are typically easy to learn, but very hard to master; most of them contain RPG elements; they are free to play; they are multiplayer!
Games of this genre are extremely popular now because they are multiplayer and all you need is your browser.
There are already many types of IO games available, from survival, battle royal, zombies and ending with racing games.
Their number grows with every day, so so check for game updates often. Who knows what will be the hit tomorrow.