The Cluster are the secondary antagonists of My Life as a Teenage Robot. They are an empire made up entirely of robotic organisms, and see themselves(mostly for its creator) as superior to biological life-forms such as humans.
The Cluster are centered on the planet of Cluster Prime, a planet completely inhabited by robots. The type of robots that make up most of the army are beetle like droids led by Smytus as their commander, and Vexus as their queen. Krakus is a known part of the Cluster, aiding them from time to time, although poorly with his inventions that fall apart or are easily destroyed.
The Cluster fell apart when Jenny came to Cluster Prime and helped liberate the people from Vexus’ hands. It is unknown what had happened to the rest of the army as every citizen on Cluster Prime has powers to match Jenny, with Vega saying, "We are no longer affiliated with those droids." It is known that Vexus, Smytus, and Krakus have been cooperating on waste asteroids, but the status of the Cluster as an entire is suspected to be complete and total destruction. There are some instances where the Cluster has used advanced androids and other types of robots in their attempts to seize Earth. In Escape from Cluster Prime, the Cluster began their invasion with very small attack bots that were invented by Krakus.
Their capabilities ranged from drilling into walls and tying up humans. Also, they electrocute when soaked with water, which proved to be problematic for the Cluster and even for its creator, although he's very problematic to water due to the fact that made himself the 'weak link' to any human aspects. When they fight against Jenny, their numbers do not serve then at any advantage. They occasionally find ways to trick her by using different types of force fields and other barriers, but they are quick to overcome and to be defeated. They have been known to run away on many occasions, not only from XJ-9 but even from their own leaders. In "Sister Sledgehammer," it's shown that the Clusters can create a energy spider web and can assimilate.
It is unknown how Clusters can produce a offspring (as Vexus did with Vega), but it is possible that they just simply build them.