IO games is a term used to describe multiplayer games that can be played directly in your web browser without the need for any plugins. They have simple rules which are easy to learn and it is also very easy to start playing. You just open your browser, open the game and after clicking one button on the main page you can start playing immediately, anytime and anywhere.
Games usually have highscore system and are based on an idea that you start small and weak and have to grow to dominate other players and your environment which makes them very addictive for massive amounts of users. Some games have tens or even hundreds of thousands of players all playing at the same time.