Death Streaks are the opposite of Killstreaks, they are meant for players who are constantly getting killed. Once you've died enough times, without any kills, your Death Streak will become active.
Description: Allows you to steal your killer's class; press the Y-button on the Xbox 360 version or the triangle-button on the PlayStation 3 version. This can give a sneak-peak at the more advanced weapons, attachments and perks, before you've unlocked them. Note: You still need to unlock the everything on your own.
Description: Works almost like the Juggernaut perk from Call of Duty 4. After you respawn, this gives you twice the amount health for 10 seconds. This can prevent most one-hit kills.
Description: Drop a live grenade just after dying. Like in Call of Duty 4, this is useful in rooms where it's difficult for the enemy to run away from the explosion. 
Description: Just like the Last Stand perk, but with more improvements. Basically this allows you to survive a normal death; you can crawl around on the ground and use your primary weapon and equipment. This is also featured in the modern warfare 3 deathstreaks list.