Offense
Offense carries the team and everyone on the team is relying on you to carry their sad little bodies to victory.
Defense
Defense help the offense sometimes but mostly serve as distractions so that the offense can destroy the other team.
Tanks
Tanks protect offense so that they can make it to the front lines and destroy the opposition.
Support
If anybody dies on your game, blame it on the support because you know how the old Overwatch saying goes, "It's somebody else's fault!" -Every Overwatch player ever.