In broadest strokes, a lot of MMOs have embraced the trinity for character design with three roles of tank, DPS, and healer. That’s fine. But even within games that have those roles there’s a lot of variation available. Final Fantasy XIV definitely has those roles as a guiding principle, for example, but your role as healer could involve lots of direct damage or lots of buffing your party members. Your DPS role might be about getting big numbers, or it might be primarily about supporting the rest of the party while you stand in the back without pants on.
Of course, those roles can expand outward, and not every game follows the exact same progression. Maybe your preferred role is doing a bit of damage whilst primarily buffing your party. Maybe you don’t really want to be a main tank, but an off-tank picking up things that get past the main tank. Maybe you just want to heal the party while throwing waves of robots at everything, as City of Heroes did not just permit but encouraged. Not every game lets you fill precisely that role, but you can often get halfway there. So what’s your preferred role in an MMO group?