UFO was a transitional band between the rock of Led Zeppelin and the metal to come in the NWOBHM.
UFO's influence was strongly felt in the '80s metal scene and they have been cited as a primary influence by Kirk Hammett of Metallica and Dave Mustaine of Megadeth among others.
UFO's early work was influenced by the space rock sound (their second album was subtitled "One Hour Space Rock") that was modestly popular at the time but the band realized the style was somewhat limited.