Cell 114

Cell 114 was the official designation for a group of sleeper agents of a species that Jack Harkness was aware of. Very little was known about them, since they didn't usually leave survivors.

The Cell was in the process of infiltrating Cardiff on Earth until Torchwood 3 became aware of them in 2008 after inexplicably fierce attacks on two burglars who had invaded the home of one of the agents, Beth Halloran. At that point they activated, and tried to set in motion a plan to explode several nuclear missiles stored in an old mine near the city. This attempt was thwarted by the Torchwood team. However, one of the agents told the team that there were more members of the invasion fleet already on Earth.

Methods
They tended to infiltrate planets, first with an advanced guard, sometimes watching for years, until they knew all they wanted to know about the species and were ready to take over.

They adapted their bodies to look like the species they were taking over (in the case of the Cardiff invasion, Humanity, at least partially trough projecting false sensor images. They also projected a forcefield less than a nanometer around their body (mainly the arm where the implant was stored) for protection.

Their main resource seems to be the aforementioned implant located in the left arm (at least when mimicking humanoids), which contained collected data and transmitted it to the main invasion fleet. It also contained at least one small bomb for sabotage or self-destruction. Furthermore the arm could be transformed into a sword-like appendage for close combat. (TW: Sleeper)