Perks are small benefits such as packages that help make your gameplay better. Examples are: ones to allow you to sprint for longer distances, ones to make you load faster, ones to make your health last longer, ones to make your bullets to penetrate harder surfaces and do more damage. There are other perks that allow you to detect enemy explosives and perks that make you invisible on the radar map (very useful if you have a silenced weapon, as a silenced weapon also doesn't report your position on the Radar. You can go behind enemy lines with a silenced M4 killing the enemy because they couldn't see you.)
There is a perk that makes you drop a live grenade when you are killed (this is the best one because you get tons of kills when people shoot you at close range, then get killed when your grenade goes off. However, experienced players see the grenade icon when a dead enemy drops it, they either running away from the thing or picking it up and tossing it away.
The Last stand perk--when you are 'killed' you don't die right away, you pull out your pistol and can take shots at your enemies for about 10 seconds. It allows you to kill the one who shot you (for short range) unless you are up against a veteran who sees you lying on your elbow and kills you.
This is a Call for Duty 4 Perks trailer
Perks are earned by getting kills in XBox live and Multiplay mode (only). The more kills you get, the more points you get toward a higher rank and better perks.