Emergency Response Team:

Sent to the station in case of Red Alert emergencies, usually requested by the crew

Their tasks range from:
-Rebuilding the Station after a catastrophe
-Neutralizing a powerful criminal
-Curing a viral epidemic when Medbay cannot do it
-Protecting the crew's evacuation to the Emergency Shuttle

They are basically Centcom's heroes, well known and liked by everyone. Protecting the crew is among their top priorities.

Deathsquad:

Sent to the station when there is no hope of rebuilding it, when there is no hope of saving most of the crew, or when the ERT has failed. Always sent by Centcom decision.

Their tasks range from:
-Wiping out a xenomorph infestation
-Killing designated targets that proved too much for the ERT (usually wizards, full power vampires...)
-Killing everyone on the station (usually in case of Revolution, or fully blown Cult...)
-Destroying the station (usually in case of Blob)
-If they aren't tasked with killing the crew, they might assist their evacuation to the Emergency Shuttle.
They will sometimes have to secure and trigger the nuke so that it detonates after the Emergency Shuttle leaves.

They are the dark side of Centcom's spec ops, they aren't officially affiliated with Centcom, though it IS common knowledge that Centcom makes use of such troops.
They are Nanotrasen's elite forces, with access to the destructive Pulse rifles and Marauder mechas.