A bleeder is a special kind of assault ship that can be created after munitions lab level 12. If it makes it to the surface of a planet, it deploys into a building. The building then converts the planet's resources into waste. It starts slow, however, it also continually upgrades itself, converting more and more resources into waste.

When a bleeder is deployed, it inserts itself at the front of the target planets build-queue. It starts upgrading itself level-by-level, again interupting anything else in the build queue.  Also Bleeders are the only building that can upgrade simultaneously.  So dropping multiple Bleeders on a target planet can be a very good idea.  Deployed Bleeders do cost a plot space.  If sufficient numbers of Bleeders are deployed on a planet a colony could gain a negative plot count.  Negative plot counts now do cause a colonies positive happiness production to switch to negative production if the plot count drops too low.

Since Bleeders do insert themselves into the build queue it is also possible to use Essentia to rush their completion.  Spies cannot steal levels off of a Bleeder so using them on a planet that you intend to hit with the Appropriate Technology mission will not end up with your spies stealing levels off of deployed Bleeders.

All you have to do to get rid of a bleeder is to demolish it. However, you have to notice it is on your planet first. And that is why it is such an insidious weapon.  Also Bleeders cannot be automatically destroyed by scripts as destroying them requires answering a captcha first.

Multiple Bleeders are able to upgrade themselves simultaneously.  Deployed bleeders cannot level up beyond level 30.

Additional Notes: Bleeders are not very effective against AI's as the AI's do catch them and remove them fairly quickly.  However if a Bleeder is left alone for about 24hours a Bleeder can reach level 18 at which point it will produce 1.3mil units of trash per hour.  An update in regards to use against AI's.  The AI logic has been altered to cause them to sometimes overlook deployed bleeders on a planet.  

Bleeders used to provide population and were a quick means to getting the largest colony.  This ability has since been removed.