One-sentence explanation:
folk music, with electric guitars and a rock beat. All the rage in the mid-
60s, with reverberations into the 21st Century, should
REM last that long. It's one of the Four Food Groups of
Rock, even though the sound of groups like
The Byrds is now rarely explicitly present, save in revivalist set-pieces like
Tom Petty's "American Girl", to name a post-60s example.
Bob Dylan snuck a weird Chicago blues vibe into his
previously-acoustic music, then started doing live gigs fully electrified, helped out by members of the Butterfield Blues Band and The Band. He had a hit with "Like a Rolling Stone"; soon Al Kooper's organ sound became au courant. The Byrds had early hits with rock versions of older Dylan tunes, borrowing George Harrison's 12-string guitar sound, another folk-rock signature.
Folk-rock became the US' answer to the British Invasion, using home-grown talent from the pre-Beatles folk scene, like Jim (Roger) McGuinn and John Phillips (The Mamas and the Papas' song "Creeque Alley" is his litany of several Greenwich Village folkies who went west and Plugged In). Jefferson Airplane took off. The British came up with its own version, with bands like Fairport Convention and Pentangle.
At the end of the 60s, led by Gram Parsons, Bob Dylan, and The Byrds, country music became a new ingredient. Folk-rock progeny: the Buffalo Springfield, Crosby, Stills, Nash, and Young, The Eagles, et al, and the laid-back L.A. album-rock of the 70s in general. Countless 80s/90s bands, from REM to The Lemonheads to Guided by Voices, touch base with the original folk-rock sound, but it's no longer a genre.