Perks are modifiers that passively affect your gameplay in The Division. Their functionality can be described as “convenience enhancers”, because in contrast to Skills and Talents, Perks don’t have to be activated manually by pressing a button or fulfilling a certain action. They are active and stay active as soon as you unlock them.
The unlocking process is directly tied to your mission progress. A list of all available Perks can be found in the Skill Calculator.