Agar.io is browser game created in April 2015 on 4chan server.
Using mouse player controls small cell which grows all the time it eat agar (small colored dots) or other cells. There is in the upper right corner statistics with top 10 players. The goal of the game is to eat as much as possible and get to the highest position. AS the cell is growing, it slows down but on the other hand the visible area is extending.
As the cell ride over significantly smaller cell, it eats the other one and grows. But if it (since some size) drive over virus, it explodes into lots of smaller parts. The cell gets speed but it is easier target for other cells.
There is pie graph in the teams mode instead of top 10 which shows rate of the three teams (red, green, blue). The strategy is also different: instead of survival of individual there is good of team much more important.
Experimental mod was created after unsuccessful introduction of some improvements. In this branch new ideas are tested before moving into main game. On 11.6.2015 the mother cell was introduced. Formerly big red circle in the middle of playground was changed into few smaller ones distributed fairly over the whole field. Mother cell creates about 10% of all agar available and it shoots it around. If some smaller cell drive into her, it dies and its mass is dispatched in the nearby area.
The viruses works also differently: instead of duplicating after seven shots of mass they move in the direction of feeding.
There are no differences between classical single player mod and party mod, the only difference is that in this mod the other players on the server are friends you invited only.
Every player needs to type some nickname (or leave it blank) and choose game mode: singleplayer, team game or experimental. He/she can also click on settings and chose some advanced settings or click on Spectate button to watch the best player. For leaving spectate mode is needed to press Esc button. The game starts by clicking on Play button.
The cells is controlled by mouse movement in squared battlefield. If the player moves the pointer to the left, the cell moves in the same direction. Because of the movement control, you cannot play Agar.io on mobile phones, because you are unable to controll the cell on your Iphone or Android device or on your tablet.
By pressing Space, the cell splits (if it have significant size) into two parts. It is handy in two situations: if the cell is chased by other player, by splitting it gets speed and it can easily escape (smaller cells are faster than the bigger ones) or just one half of the cell is eaten by the enemy. More often splitting is used for eating some others cell. If the cell is more than two times bigger than some other in the nearby, player can split by pressing Space in the direction of movement. The initial speed is very high and if he/she doesn't miss (and is sufficiently bigger), the other cell is eaten. These two parts will merge after some time. The fact the merge is coming can be observed by penetrating one part into another on touch.
Last button is the "W" key. Thanks to it the player can fire small part of his/her mass into the space. There is few functions of this: firstly you can feed someone else, which is very useful in the teamplay mode. It is often used when one cell is chased by other one from different team: by feeding you can make your team mate grown and change the positions of predator and prey. The efficiency is only 80%, which means one player gets just 80% of the mass given away by the other one. In the singleplayer (as well as team mode) it is more often used for duplicating viruses, which destroys your enemies (which you can afterward eat). If one virus is fed it starts to grove and then it duplicate in the direction of feeding (you need to press "W" 7 times). It is also used for communication (by shooting mass onto another cell you can proclaim peace or ask for help to destroy some other cell. Sometimes there are symbiotic partnerships created where smaller helps the other one and the bigger one protect and feed his ward).
Tips and tricks
If you are small and want to drive in between two big ones, don't be afraid. Non of them will split because it is very dangerous in face of other enemy. It also works in team mode if you try to escape inbetween cells of two enemy teams. You can also escape from two cells of the same colour or two parts of the splitted cell. The place for maneuvers is really small and the danger is very high therefore it should be used just in case of real emergency.
If you are playing team game and your partner is in danger, feed him/her. He/she overgrow the enemy and swallow his/her cell. Another useful help is to explode enemies cell when it maneuvers around virus. If someone helps you in this situation or during group hunting, it is gentle to thank this player by returning adequate amount of mass by pressing "W". The collective mode grow as well as cooperation which is useful not just for team, but also for individuals in the team as there is no chance to survive in the environment full of enemies.
In singleplayer mod there is few skins which differs from just colored cells. You cannot create your own skin but, only use the existing ones by using right nickname. The overview can be found here (also here with images) and ask for a new one here.
As of July 2015 the statistics was added. At the end of the game after you die there is window with statistics about how many cells you ate, how long have you been in leading and others.
You can disable the statistics window in settings.
Until July 2015 there was no option to use stable account. Starting that month you can log in using your Facebook account. If you don't like if for any reason you can still just type in your nickname a play anonymously.
At the end of May and beginning of June a lot of players complained of lags. The problem was in overloaded servers and it was necessary to wait for other players to leave (or for author to buy new servers or release the game under free license for other admins to run the game on their servers).