Each class has one advantage over another class, and one disadvantage against another class.
Here is a full chart of the class bonuses:
You can view the enemy class breakdown within a mission by selecting the reticle icon on the bottom-right corner of the screen above the quit mission icon.
On the pop up that appears, you can view how many enemies of each class type are in the mission, as well as determine the enemy class type of the boss by the skull icon.
