A Terminator model T-100 without its 'skin'
Terminators are robots built in the future that were designed to defend strategic bases and attack targets. Due to the idiocy of the people who built them, they ended up going rogue, and now scour the Galaxy attacking innocent freighters and scout ships.
The League Military is under orders to destroy Terminator ships wherever they see them, and Terminator sightings have been on the decline. However, occasionally a freighter will go missing and, upon further investigation, the marks of the Terminators will be all that is left behind.
Terminators come in different forms. T-100s are humanoid robots that sometimes use an outer layer of 'skin' to disguise themselves. These are the most common form of Terminator, and are seen often in Terminator encounters. They are commonly used as drones and warriors by the Terminator forces. They are difficult to kill but some species have strengths that outmatch even that of a Terminator, for example, Hunters and Cybermen. It is theorised that there must be models named T-200 and T-300 etc, but some same these exist in Terminator society as engineers and pilots rather than warriors. A Terminator Tank was once seen with the insignia 'T-220', suggesting these types of Terminators are not humanoid but are in fact larger machines.
T-850s are more advanced forms of Terminators programmed with emotions. These are commonly seen commanding ships crewed by Terminators and leading their attacks. Few have ever been killed, and those that have been killed self-destruct before their bodies can be studied. A Dalek strike team consisting of eight Elite Dalek warriors once went against a T-850, and only garbled messages and smouldering Dalek wrecks were found when the dust settled.
Another form is the T-1000, a Terminator that is made of an advanced polymer that literally allows it to turn into a liquid and back again. This allows them to change their disguises often, and slip through tiny gaps. It also makes them very hard to kill. Only one has ever been confirmed as killed, since it went down with its ship. A UNSC battlegroup went against the T-1000's ship in battle, and three of the human ships were destroyed by boarding craft filled with T-100s before the Terminator ship was obliterated by MAC blasts.
The most advanced form of Terminator, however, is the T-X. She has the liquifying powers of the T-1000 combined with the tactical advantage of the T-850, and there is only one in existance. She is thought to be the leader of all the Terminators, but even this is in dispute. Others believe she is a Lone Wolf, going against even her own kind and believing herself to be supreme.