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 broken the law but had escaped punishment. After finding them near the end of their respective timelines, the crew would administer their own punishment - usually in the form of some painful torture before death.
The Doctor, Amy Pond, Rory Williams and River Song encountered the Teselecta in Berlin, 1938 where they accidentally stopped it from killing Adolf Hitler.
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. (DW: Let's Kill Hitler)