The work is an early example of Pärt's tintinnabuli style, which he based on his reactions to early chant music. Its appeal is often ascribed to its relative simplicity; a single melodic motif dominates and it both begins and ends with scored silence. However, as the critic Ivan Hewett observes, while it "may be simple in concept...the concept produces a tangle of lines which is hard for the ear to unravel. And even where the music really is simple in its audible features, the expressive import of those features is anything but." A typical performance lasts about six and a half minutes.
The cantus was composed as an elegy to mourn the December 1976 death of the English composer Benjamin Britten. Pärt greatly admired Britten, whom he described as possessing the "unusual purity" that he himself sought as a composer. Pärt viewed the Englishman as a kindred spirit; however, he gained access to the latter's music only in 1980, after emigrating from Soviet Estonia to Austria, four years after Britten had died. When Britten died, Pärt felt that he had lost hope of meeting the only contemporary composer whose musical outlook, he believed, resembled his own.
While Pärt is known primarily for his religious music, Cantus is a fully secular work, in that it forms a spare lament to a fellow composer not based on biblical texts. It is perhaps Pärt's most popular piece, and a 1997 recording by the Hungarian State Opera Orchestra conducted by Tamas Benedekand has been widely distributed. Due to its evocative and cinematic feel, the piece has been used extensively as background accompaniment in both film and television documentaries.