A rock and roll/jazz fusion group, they put out their best stuff 1968-1972. Spirit is:

Randy California: Vocals, Guitar
Jay Ferguson: Vocals, Percussion
Mark Andes: Bass
John Lock: Keyboards
Ed Cassidy: Drums, Percussion

They’re pretty good, though not the most well known band in America. Sound sort of like a mix between Traffic and Moby Grape – a lot of noodly jazzy bits for people who like that sort of thing, and a lot of good old fashioned electric rock and roll. Interesting tid-bits:

The intro to Stairway to Heaven by Led Zeppelin is an utter rip from Taurus, an instrumental by these guys. See Stairway to Heaven for more info.
The drummer is the stepfather of the lead singer. Kooky.

For some good tunes, check out Fresh Garbage, the aforementioned Taurus, or I Got a Line on You.