Agar.io isn't a game you have to download, although if you want to play it on your phone or tablet, you'll need to download the free app. It can be played in the browser just as well. It's a game where you enter your name, and then immediately start guiding your large, bordered dot over countless smaller "food" dots, while avoiding or eating other, smaller players as well. It's simple, but fun.
And how there are other players
When you first start playing Agar.io you'll often lose your progress to a much bigger dot ("cell" in the game's parlance) who gains a huge burst of speed and quickly overtakes you. But once you're bigger than some other players yourself, you can return the favor if you so choose. Other players are at the heart of the game, really: if not for them you'd be moving across dots without much strategy all day. As it is, you'll need to move to avoid or trap them, use the sporadically-placed cover to surprise them, and otherwise interact with Agar.io's color cast of players. It does bear noting that some usernames (and even skins) can be offensive, and if a particularly unfortunate player is around, you might consider playing later as a result.
Agar.io is as simple as it is fun. However, in spite of that simplicity, there's a rough edge to it. You lose all of your progress when an enemy player defeats you, and that can set you back countless dots worth of time. If you register, which most players should, you do gain experience levels over time as a form of permanent progress, but these don't do much on their own. The game provides a few ways to make staying alive and reaching the leaderboards (which show the biggest dots currently in-game) easier. These are all accessible by Coins. Coins can be obtained for free on a very limited basis, but it is easier and more common to buy them. They can provide time-based boosts, either one hour or twenty four, to your starting size or the experience levels you gain. They are also needed to buy skins, which change your character's appearance. Some skins, which require high levels, even provide special abilities such as increased speed. You don't need to buy coins to win or enjoy yourself, and the fact you lose your size eventually means it's about as permanent as buying a nice lunch. But the option to drop a fiver on the game will always be there.
A good time, quickly over
Overall, even though other players can be mean or a little racy, Agar io is still a good time during one of those inevitable waits-in-line. It's a perfect mobile game, and a decent desktop distraction while you're waiting for something to happen at work. That's the game's ultimate virtue, and outside of earning a little on the side from microtransactions, it doesn't aspire to much else.