The Aliens originally hail from the planet Alien, apparently located somewhere in our own Milky Way Galaxy. Exiled from their homeworld, they arrived on Earth an unspecified number of years ago, and have been quietly operating under Head Alien's leadership to gather up the artifacts of the Martian Empire.
During the 1980s, the Aliens joined forces with Linda Walkerton, abducting human children and grafting superpowers onto them in an effort to create a fighting force that would protect Earth from a future Martian invasion. This partnership did not last, and when Dargon Chesterfield created the organization SEMME to retrieve and manage the abductees, SEMME's missions regularly brought them into conflict with the Aliens' various schemes.
The two factions put aside their differences again during the Martian invasion of Earth, when humans and Aliens briefly fought alongside one against a common foe. After the invasion was halted, the remainder of the Alien force gathered under the leadership of Monkey Master. Their ultimate fate is still unknown.
The Aliens are extremely short by human standards, standing about three feet tall. They are neither particularly strong nor sturdy; as Alan Rees noted, their their weak spot is pretty much their entire body. It can be assumed that Earth's atmosphere is toxic to them, requiring them to wear all-concealing containment suits at all times.
The Aliens' other notable anatomical quirk is the fact that they only have one "normal" hand; the other hand is actually a pincer.
The Aliens have powerful racial telekinetic abilities, but these powers are blocked by their containment suits.
The Aliens' native technology is controlled by telekinesis, which presented immediate issues upon their arrival on Earth. To compensate for this, the Aliens co-opted a variety of Martian devices and discovered ways to weaponize human pop culture, most notably The Sound of Music.
The Alien seem to have particularly advanced knowledge of genetic engineering and modification, knowledge that was used to create the abductees as well as artificial lifeforms like the Giant Mutant Frosted Honey Bun.