IO games are browser-based multiplayer games that use '.io' domain extensions. The name comes from the British Indian Ocean Territory domain — but players just know them as the most addictive multiplayer browser games ever made.
Agar.io launched in 2015 and created the entire genre. Its simple premise — grow by eating smaller players, avoid larger ones — spawned hundreds of imitators and innovations. Slither.io, Paper.io, and Krunker.io each added new mechanics while keeping the core accessibility.
Start with Slither.io — it has the most forgiving learning curve. Use the coiling technique to trap smaller snakes. Avoid larger snakes at all costs early on. Once you reach top 10, become aggressive.