Perks provide always-on passive benefits, just like Rig traits.
Unlike traits, however, you can take a lot more perks on a build—up to six maximum. Taking that many pretty much limits you to a gun and nothing else, so usually from two to five perks depending on your build is preferable, unless you’re working on odd challenges.
Perks define your build, and you should take care to choose a perk setup that combines well with your Rig choice and the game mode you are playing. Some perks are better suited for objective play, some for aggressive rushing, and some for defense.
You can take up to two perks in each of the three categories, but the second perk costs two points instead of one. Only double up on perks from the same category if they’re vital to your build (such as taking Dead Silence and Hardwired on a stealth-focused class).