Think of a tactical shooter in the vein of Battlefield minus the vehicles but with the addition of free-roam scaling of any environment you're in.
There's customization for your personal character (clothing, tattoos, hairstyles, facemasks, etc.) as well as the weapons they use and the gear they carry (if you play as a Heavy-type Soldier, you can carry a huge Mini-gun and dispense ammo to teammates for example); all of which can be earned through XP while playing.
Also, depending on the level you're on and what class type you have, you can choose to do extra objectives for XP points and to help your team win (you can set up turrets as an Engineer to help cover your buddies in a firefight).