The Briori are an alien race from the Delta Quadrant. In 1937 they kidnapped a group of humans from Earth and took them back to an unnamed planet in the Delta Quadrant. Some (including Amelia Earhart) were cryogenically frozen while the rest were used as slaves.
Eventually there was a revolt and the humans drove the Briori off the planet, claiming their technology and making the planet thier own. They developed a high level civilisation and were discovered by the crew of the USS Voyager in 2371. (VGR: "The 37's")
The Briori themselves have not been encountered by Starfleet and it is not known how they travelled the huge distance between the Delta Quadrant and Earth.