The Teselecta was a Justice Department Vehicle, staffed by a crew that had been miniaturised by a compression field. It could travel through time, and was used to locate people who were deemed to have committed crimes but had escaped punishment. After finding them near the end of their respective timelines (so as to avoid changing history), the crew would administer their own punishment - usually in the form of some painful torture before death.
History
The Teselecta was manned by a staff of four hundred and twenty-one people. They could change its appearance to any being. At the time the Eleventh Doctor, Amy, Rory, and Mels crashed through Hitler's office, the Teselecta was trying to deliver punishment to Hitler, but got him in the wrong time period. It wasn't until they discovered Mels, who had regenerated, revealing herself as Melody Pond, that they stopped trying to go after Hitler, and tried to deliver her punishment, for their records indicated that she would be the one to kill the Doctor. They were thwarted by Melody's parents, Amy and Rory, who deactivated the devices that protected the crew against the antibodies. They were forced to teleport to a spaceship above the Earth, abandoning the Teselecta. The Teselecta was used to show Melody that River Song, the friend of the Doctor and his companions was what she would later become. The Doctor also extracted data from it regarding his death and let it reveal the Silence's purpose. (DW: Let's Kill Hitler)
The Tessalecta returns in The Wedding of River Song and ends up taking the shape of The Doctor to save him from the Astronaut which turns out to be River Song
Features
Shapeshifting
The Teselecta could resemble anyone or anything whose likeness it has scanned. During its time in Berlin, 1938, 6018 took on the forms of a janitor, a Nazi official, a Nazi soldier (both with and without a motorbike), as well as Amy Pond and River Song. A member of the crew stated that it had once taken the form of Rasputin, but with green skin.
Miniaturisation ray
The Teselecta was equipped with a miniaturisation ray, which it used to miniaturise people and transport them into the Teselecta. Compression fields were used to keep people miniaturised.
Antibodies
The interiors of the Teselecta were defended by the Teselecta Antibodies. Anyone without the proper privileges was killed. (DW: Let's Kill Hitler)