Shared Features

With some exceptions, all classes have the following stats and skills:

600 Hit Points
500 Energy
25% innate damage reduction armor
1200 sight radius
4.2 HP and Energy regeneration/second
900 range, 1 second cooldown, 50 damage assault rifle
285 move speed

Skills:
Kick - When an enemy closes on an operative, hoping to get the advantage, they only get a boot in the face. Kick knocks back enemies and stuns them for a short time. This skill is of absolute importance for your survival. Here's an analogy of someone who would use kick correctly. Here's an analogy of what to expect if you stand around auto-attacking and don't ever use kick!

Sprint - Operatives can push themselves to bursts of speed using jet thrusters in their boots, allowing for escape or reaching important destinations more quickly. This skill is also of absolute importance for your survival.

Though you can play a game with everyone being the same class, realize that all the classes are specialized towards certain challenges and weak against other challenges in the game. You can make it through with such a group, but having a diverse class composition means that you're less likely to have a tough time in certain spots as you progress.

On the other hand, the skill and item system is set up such that you can have 2 or more people of the same class with completely different strengths and weaknesses! So don't worry too much if your team has a few people playing the same class.