There are four different classes to choose from in the game, and each one provides a different benefit on the battlefield.
Assault Class - Your bog-standard soldier, the Assault Class excels at using standard shotgun and assault rifle weapons in close-quarters combat.
Heavy Class - If you're struggling to take out obstacles and enemy clusters, consider making use of the Heavy Class. Rocket launchers and explosives are the order of the day here.
Sniper Class - If getting up close and personal is causing carnage, make use of the Sniper Class to get an advantage. These ranged experts can deliver blistering firepower from distance - allowing you to take out the enemy and live to fight another turn.
Support Class - The Support Class excels at two things - providing medical aid to injured squad-mates, and offering logistical support to help the team move more stealthily through dangerous environments. Protect these guys at all costs.