Game modes are styles of gameplay in surviv.io. Currently, there are three game modes available; solo, duo, and squads. Initially, you could only play solo, but duos was added in Patch 0.2.0 on January 17, 2018. Squads were added in 0.3.0. Trios is also possible but you will have to make a custom game with 2 other buddies with auto-fill off and on squads.
Solo is the core game mode of surviv.io, as well as the first to be available. It is also arguably the most popular game mode of surviv.io due to its ease and complexity. Teaming (partnering with another player) is possible but discouraged as it displays a lack of expertise and may lead to easy death.
Duo, commonly referred to as Duos, is the second game mode to be added to surviv.io, added in patch 0.2.0. The game is nearly identical to solo mode except that there are two players per team. Cross-teaming is rare in this mode. It is possible to play with a friend of yours or someone random.
Squad, popularly known as Squads, is the third (and newest) game mode to be added to surviv.io, added in patch 0.3.0. Squads is solo in concept, but with a total of four people on a team. It is more difficult to play with random people since the range of expertise spans from professional to rookie. Frequently special events occur in this game mode, typically featuring increased flare gun drops.