Overwatch is classified as an Multiplayer Online Battle Arena but this honestly will not give you an idea what you are really playing.
The break down for Overwatch: 6vs6 Team objective based game that gives you the ability to swap in and out of heroes at spawn or after a death.
To ensure a team victory players should consider swapping to fill key roles as tank and or Support. These provide a strong base team.
Although tank and support are not always played you could see a victory without them but it would take much more effort and map intelligence
Players should consider different heroes for different maps to fully utilize the full potential of the hero.