Variously spelled doo wop, doo-wop, and doowop, the genre first appeared in 1950 and remained popular until the advent of rock and roll (roughly 1955-56), after which it began to fade. But it enjoyed a revival in 1959, spurred by the release of oldies collections. It faded again after the "British Invasion" of 1964. Its second revival was in the early 70s.
Doo wop is a subgenre of vocal group harmony. It's a style of smooth harmony singing without any, or with minimal, instrumental accompaniment, almost exclusively sung by male groups, and is notable for its simplicity, innocence, and joyousness. While there is no consensus as to what constitutes a doo-wop song, the typical group featured a lyrical soprano or falsetto voice, a deep rumbling bass voice, and two-or-more complementary harmony from the rest of the group.
The name "doo wop" derives from the practice of having the lead vocalist sing the lyrics against a backing of nonsense syllables from the other members of the group. It was first practiced by street-corner groups in the inner cities, some of whom went on to make hit records, though most were one-hit wonders. African-Americans originated and dominated the genre, but the late 1950s-early 1960s also saw the rise of Italian doo-wop groups, including Dion and the Belmonts, The Capris, the Mystics, and the Duprees. Two racially integrated groups were The Del-Vikings and The Crests. Only a few all-female groups became popular, most notably the Chantelles, the Hearts, and the Shirelles.
Doo wop had its roots in rhythm-and-blues and gospel music; indeed, many of these groups are also classified as 1950s-60s R&B; or soul. Some say doo wop is a form of R&B;, others say it's a form of pop music, and yet others say it was the closest rock style to mainstream pop in the early 1950s.
There are at least hundreds if not thousands of doo-wop groups, so feel free to submit names not on the list, though I've included every group that's in the Doo Wop Hall of Fame (doowophof.com, which also contains a list of doo wop websites).
In music, if you like it, it's good--Duke Ellington.