Perks have the following effect.

Marathon: Unlimited sprint.
Sleight of Hand: Faster reloads.
Scavenger: Resupply from dead enemies.
One Man Army: Change classes at anytime at the loss of a secondary weapon.
Stopping Power: Increase bullet damage against enemy players.
Lightweight: Move quicker.
Hardline: Killstreaks require one less kill.
Cold Blooded: Undetectable by UAVs, air support, sentry guns and thermal vision.
Danger Close: Increased explosive weapon damage.
Commando: Increased melee distance.
Steady Aim: Increased hip-fire accuracy.
Scrambler: Jams enemy radar near you.
Ninja: Invisible to heartbeat sensors.
SitRep: Detect enemy explosives and tactical insertions.
Last Stand: Fall to the ground and use your secondary weapon until you bleed out.
Bling: Two primary weapon attachments.