Diep.io was one of the first in the wave of ".io games"--massively multiplayer PvP browser games that pit players against one another in a relatively small arena, generally with the goal of becoming the biggest.
While .io games themselves are generally straightforward, having the player dodge and kill other players while they themselves grow larger, Diep.io does things a little differently. Players still have to compete against one another to get larger but there is an increased focus on farming food over fighting.
Along with the increased focus on farming is the game's RPG-style stat point and class system. Instead of arbitrarily growing larger as players eat generated particles and the remains of other players, they must shoot food to gain EXP towards leveling. With each level comes more stat points, and at set levels come class choices.
Farming itself in Diep.io is an active affair. In the better-known Agar.io and Slither.io games players do not have much in the way of pure offensive abilities. Players can split to eat other players in Agar.io and in Slither.io they can boost to try to cut enemy players off, but Diep.io is more akin to shoot'em up games. Players control a bullet-shooting tank and must dodge stray bullets to survive.
Maneuvering the tank is done with the arrow keys on the keyboard and aiming is done with the mouse for optimal accuracy and speed. Some classes, such as the gimmicky Overlord and Necromancer, have additional keys to use to make the most of their unique abilities.
Between the shoot'em up-style gameplay and the leveling system players must farm food via shooting it and leveling up before they can hope to compete and survive against others on the map. Farming is done by shooting automatically-generated food particles around the map, which spawn in four different types.
Yellow, square food gives the lowest EXP and is best for very early levels. Red triangle food gives slightly more, and blue hexagons give more than both combined. The fourth food type, giant blue hexagons, give a massive amount of EXP but themselves have a very large HP pool and are generally found in blue hexagon clusters, which are protected by homing AI bullets.
At level 15 players have the ability to choose their first class change. The initial choices are:
Each class branches off into more powerful classes at level 30, each one with its own branching paths. Some classes have even more powerful options when they reach the maximum level of 45.
The general goal of Diep.io is to reach the top of the leaderboards, which are sorted by score. Players' scores are raised as they kill food and other players and can be seen below their names. Killing players with higher scores nets higher score rewards.