Source: The Empire Strikes Back
On Cloud City, Ugnaughts operate the carbon-freeze chambers that are typically used to freeze Tibanna gas but can also freeze living matter. Ugnaughts work in Bespin's Tibanna gas processing plants or as general laborers in the city.
These characters were mainly seen in The Empire Strikes Back during the carbon freezing scenes with Han Solo, and when C-3PO was shot by Stormtroopers.