Role
Each class in Echoes has a distinct role to play within a fellowship. It may or may not be similar to the role you play in solo battles. A fellowship operates with a single purpose, but with many different parts working together to reach that goal. A coordinated effort will succeed; chaos will usually fracture the fellowship and lead to failure.
In general, class roles fall into categories that take best advantage of class skills. These categories are most often described as DPS, Healer, Support, and Tank. While each class has unique strengths within a fellowship, that doesn't mean there is no room for improvisation.
Damage/DPS
The DPS (damage-per-second) is the fellowship's main damage dealer. They provide the offensive punch for the fellowship to quickly plow through enemies. The two classes that naturally fill this role are Champions and Hunters.
Healer
The Healer's role in a fellowship is to keep their fellows' morale (health) high. While Captains and Lore-masters do have some skills that bolster morale, the only true dedicated healer in the game is the Minstrel.
Support
The Support role is the most varied within a fellowship. Burglars, Captains, and Lore-masters are typically considered Supports. While each of the other three roles have fairly clear metrics to tell how well they are performing their job, a good Support can look like a lot of different things. Maybe a Burglar will decide to use their Stealth to sneak ahead and scout the lie of the land, providing crucial pathing information to the rest of the fellowship which allows them to avoid the strongest enemies. A Captain can support their fellowship by providing crucial buffs to their allies, making them strong enough to take on enemies which would normally outmatch them. A Lore-master might share their power with the group's Minstrel, allowing the healer to save the day at the end of a long fight.
Tank
The Tank's responsibility is to protect the rest of the fellowship from harm. He accomplishes this by holding the enemy's attention (aggro) on himself and surviving while everyone else does their thing. While the Champion can fill this role if needed, the Guardian has the best defenses and aggro management abilities in the game and is the most naturally suited to the role.