A requiem usually means the “Mass for the Dead”. It takes its name from its introduction: “Requiem aeternam” (eternal rest). The “happier” parts of the normal mass are omitted (the Gloria and Credo) and a 13th century hymn “Dies Irae” introduced. The rest of the text is generally followed, except for small changes to the Agnus Dei.
The medieval service was accompanied by plainsong but - as was the case with the normal mass, and every other musical part of the liturgy - the late medieval-early renaissance development of polyphony saw more sophisticated and extended settings, many of which have come down to us. The first polyphonic setting – though of four parts only – is by Ockeghem, about 1470. The first full-scale orchestral/choral/vocal requiem written with half an eye on the concert stage was Mozart’s (completed by Süssmayr) in 1791. In the intervening period 100s had been written.
Notable requiems of the 19th century were written by Cherubini (who wrote two), Hector Berlioz, Franz Liszt, Giuseppe Verdi, Camille Saint-Saens, Anton Bruckner, Antonin Dvorak and Gabriel Faure. Whilst most were conservative works, (Fauré’s is particularly restrained) those of Berlioz and Verdi are in an oratorio style that employ enormous forces and mirror the composers’ own dramatic inclinations.
The three notable requiems of the twentieth century are by Duruflé; a conservative work, Benjamin Britten – the “War Requiem”, with traditional Latin texts interspersed with the poetry of Wilfred Owen, is one of the great works of any genre of the century - and (by contrast) Andrew Lloyd Webber, which is, not surprisingly, lightly lyrical.