There are three factors that make a class. They depend on your selection of sub-classes. There are 9 sub-classes, they are grouped by 3 and you can only choose one from each group. You must select a sub-class from each group.
First group - classes by M.O.
Assault class - the main combat core of each modern tactical combat squad. Usually armed with assault rifles, submachineguns and pistols. Small arms experts, the only sub-class to be able to dual-wield weapons.
Stealth class - this usually long-range tactical suppression unit is trained in the ways of the shadows, camouflage and silence. Prefer long-range weapons, but also proficient with scoped machine-guns.
They usually perform diversions to confuse the enemy and divert the attention from the main assault.