Enemies:

Mite

Part of the first wave of an attack force. They are used to test defenses. Born out of eggs laid by a queen, they start off small and keep growing. They are little more than mobile eating machines attacking in packs and are used to sample DNA for later analysis for the Skavs.
Crawler

Used to sneak about a ship gathering information. They prefer small dark holes to hide in and use there radar like abilities to spy on the enemy.
Skav
Closer to the original genetic make-up of their makers than any of the aliens, Skav's are part of the final wave of attack. The are mobile genetic processors, they consume and break down their victims and return with results.
Skav Leader

A more powerful Skav, they act as a nexus for all the aliens in the area, coordinating attacks and ensuring orders are carried out. Their primary goal is to get the Worm Hole Devices.
Hunter

I think the name says it all. Specializes in tracking and killing prey. To destroy is their one function, something they do with great cruelty and viciousness. Created using the DNA of several predator species.