Agar.io is the game that started it all, being the first real io game it defined the genre with its simple core mechanics. You control a small circle/cell/orb and you collect points that will make you bigger and therefore better. The mission is to eat other, smaller orbs than you, in order to become the biggest in the game. But watch out for bigger ones, they can consume you, which of course kills you instantly. The game seems really simple at first glance but playing it longer will show you the depth of this fun multiplayer game. So for example, you're also free to team up with other orbs, to help each other out and climb to the top of the ladder.
Simply point your cursor to where you want to move and your orb will go there. Press space to split yourself into two parts which also gives you a quick boost and makes you faster. Use the space bar ability for catching much smaller orbs. You can also hide behind green circles that have spikes around them, but you have to be smaller than them. If you hit them when you're bigger you split up into a thousand pieces, which of course should be avoided. But even if that happens, you're split up, and you don't know if you will ever be united again, just be patient, after a few minutes it'll happen automatically and you're reassembled again. The last ability you're given is pressing the W key, it will shoot out a very small part of you, which can be used for either bait or to help your friends.