Agar.io is a online multiplayer game released in 2015. Made by a Brazilian developer Matheus Valadares.
Gameplay is to control a cell, which starts as a small dot on a grid. The main goal of Agar.io is to grow as big as possible by eating smaller cells and avoiding bigger ones. Bigger cells can eat you if they touch you. So, you need to be aware of your surroundings.
Easy to control, just move your cell with your mouse cursor. On mobile, use touch.