Answer the question at the end by quoting:

A musical ensemble, also known as a music group or musical group, is a group of people who perform instrumental or vocal music, with the ensemble typically known by a distinct name. Some music ensembles consist solely of instruments, such as the jazz quartet or the orchestra. Some music ensembles consist solely of singers, such as choirs and doo wop groups. In both popular music and classical music, there are ensembles in which both instrumentalists and singers perform, such as the rock band or the Baroque chamber group for basso continuo (harpsichord and cello) and one or more singers.
Five-piece bands have existed in rock music since the development of the genre. The Beach Boys, The Rolling Stones (until 1993), Aerosmith, Def Leppard, AC/DC, Oasis, Pearl Jam, Guns N' Roses, Radiohead, The Strokes, The Yardbirds, 311 and The Hives are examples of the common vocalist, lead guitar, rhythm guitar, bass, and drums lineup whilst other bands such as Judas Priest have two guitarists who equally share lead and rhythm parts. An alternative to the five-member lineup replaces the rhythm guitarist with a keyboard-synthesizer player (examples being the bands Journey, Elbow, Dream Theater, Genesis, Jethro Tull, The Zombies, The Animals, Bon Jovi, Yes, Fleetwood Mac, Marilyn Manson and Deep Purple, all of which consist of a vocalist, guitarist, bassist, keyboardist, and a drummer) or with a turntablist such as Deftones, Hed PE, Incubus or Limp Bizkit. Alternatives include a keyboardist, guitarist, drummer, bassist, and saxophonist, such as The Sonics, The Dave Clark 5, and Sam the Sham and the Pharaohs. Another alternative is three guitarists, a bassist and a drummer, such as Foo Fighters, Radiohead, and The Byrds. Some five-person bands feature two guitarists, a keyboardist, a bassist and a drummer, with one or more of these musicians (typically one of the guitarists) handling lead vocals on top of their instrument (examples being Children of Bodom, Styx, Sturm und Drang, Relient K, Ensiferum and the current line up of Status Quo). In some cases, typically in cover bands, one musician plays either rhythm guitar or keyboards, depending on the song (one notable band being Firewind, with Bob Katsionis handling this particular role). Other times, the vocalist will bring another musical "voice" to the table, most commonly a harmonica or percussion; Mick Jagger, for example, played harmonica and percussion instruments like maracas and tambourine whilst singing at the same time. Keith Relf of the Yardbirds played harmonica frequently, though not often while also singing. Ozzy Osbourne was also known to play the harmonica on some occasions (i.e. "The Wizard" by Black Sabbath). Vocalist Robert Brown of lesser known steampunk band Abney Park plays harmonica, accordion, and darbuka in addition to mandolin. Flutes are also commonly used by vocalists, most notably Ian Anderson of Jethro Tull and Ray Thomas of the Moody Blues, though these are difficult to play while singing at the same time. A less common lineup is to have lead vocals, two guitarists of varying types and two drummers, e.g. Adam and the Ants.

how do they differ if at all?

Answer the question at the end by quoting:

John Glascock (2 May 1951 - 17 November 1979) was the bass guitarist, backing vocalist, and occasional lead vocalist for the progressive rock band Carmen. He was also the bass guitarist and backing vocalist for the progressive rock band Jethro Tull from December 1975 until August 1979. He died in 1979, at the age of 28, as a result of a congenital heart valve defect, which was worsened by an infection caused by an abscessed tooth. In a September 1978 'Guitar Player' interview, Ritchie Blackmore had said "...
In 1973 he joined Carmen. With them, John Glascock played bass guitar, sang backup vocals, occasionally played synthesizers, and even sang lead vocals on a few songs. These songs include "The City", "Dancing On a Cold Wind" (sharing lead vocals with David Allen), and "The Horseman" (sharing vocal duties with all of Carmen), all from the album Dancing on a Cold Wind (1975). He also sang on the track "High Time" from the album The Gypsies (1976). Glascock's basslines in Carmen were often technical and unusual. They were key to Carmen's unique sound. Unlike his work in Jethro Tull, Glascock utilized effects in his sound while with Carmen, like a fuzz pedal in the song "Viva Mi Sevilla". Glascock had only one writing credit on the first album recorded by Carmen, Fandangos in Space (1974), the short track "Retirando". He shared this writing credit with the rest of the band. His writing expanded greatly by the time of the second album, Dancing on a Cold Wind (1975). On this album he had a writing credit for three songs, "Viva Mi Sevilla" (which he shares with the rest of the band), "Purple Flowers" (which he shares with Roberto Amaral), and "Remembrances" (which he shares with the rest of the band). His writing had reached a climax by the time of Carmen's third and final album, The Gypsies (1976). On this album he wrote "High Time" by himself, and sang lead vocals on it. He and the rest of the band also share a writing credit on the instrumental track "Margarita".

what role did he play in the carmen band?
John Glascock played bass guitar, sang backup vocals, occasionally played synthesizers, and even sang lead vocals on a few songs.