Agario is the IO game that started it all. Literally. It became so popular so quickly that it influenced many of the IO games to come. Start as a tiny "cell," eat orbs to grow larger, and then consume other players who are smaller to grow even more and top the scoreboard..

Guide your cell with your mouse. Space splits your cell into two equally-sized cell (repeatable), and W ejects mass in order to give yourself a speed boost.

The general strategy is to avoid being eaten, but it gets pretty complicated from here on out. There's a lot to be said to how much you boost (don't waste all your mass going faster - you'll just shrink indefinitely). Splitting also takes finesse. It'll give you an advantage over cells you're hunting, but it will also leave you vulnerable until you recombine.