There are two characters available for each class. While broadly similar in their role, each features their own abilities and strengths.
The Assault class is a role that is a staple in many squad-based shooters, and functions as both a massive damage dealer and damage absorber, called "high-DPS" and "tank", respectively. The Assault class comes with a large health pool, and paired with some powerful weapons, is the ultimate juggernaut.
Trappers are the experts in tracking and pinning down Monsters, with a range of specialized abilities for the job.
Medics are both capable healers and capable warriors, able to not only heal and resurrect comrades, but also deal area-specific damage due to their vast knowledge of anatomy and alien bio-structure.
The Support class is a formidable kit with many defensive and heavy offensive capabilities. They can boost their teammates' output, help them escape attacks, and of course dish out some damage of their own.