Imagine Joan Baez singing over the best laidback folk-rock from the early '70s, and you'll know a bit about the majesty of Fairport Convention. You won't have the whole picture -- the Convention made stuff no one else ever did -- but you'll have a good part.
For a fuller vision, check out Meet On The Ledge: The Classic Years, which records must-have sides from 1967-75, the period during which the band crystallized its reputation as England's greatest folk band.
Such a term was nice of critics to give, of course, but it's loaded. Folk here means that members had a clear understanding of indigenous music traditions in America and Great Britain. They just took them in directions not always valued in the conservative folk world. Fairport was twangier and more psychedelic, and it gave traditional music a much-needed kick in the pants.