A refrain (from Vulgar Latin refringere, "to repeat", and later from Old French refraindre) is the line or lines that are repeated in music or in poetry; the "chorus" of a song. Poetic fixed forms that feature refrains include the villanelle, the virelay, and the sestina.
In popular music, the refrain or chorus may contrast with the verse melodically, rhythmically, and harmonically; it may assume a higher level of dynamics and activity, often with added instrumentation. Chorus form, or strophic form, is a sectional and/or additive way of structuring a piece of music based on the repetition of one formal section or block played repeatedly.
In music, a refrain has two parts: the lyrics of the song, and the melody. Sometimes refrains vary their words slightly when repeated; recognizability is given to the refrain by the fact that it is always sung to the same tune, and the rhymes, if present, are preserved despite the variations of the words. Such a refrain is featured in "The Star-Spangled Banner," which contains a refrain which is introduced by a different phrase in each verse, but which always ends:
A similar refrain is found in the "Battle Hymn of the Republic," which affirms in successive verses that "Our God," or "His Truth," is "marching on."
Refrains usually, but not always, come at the end of the verse. Some songs, especially ballads, incorporate refrains (or burdens) into each verse. For example, one version of the traditional ballad "The Cruel Sister" includes a refrain mid-verse:
(Note: the refrain of 'Lay the Bent to the Bonny Broom' is not traditionally associated with the ballad of "The Cruel Sister" (Child #10). This was the work of 'pop-folk' group Pentangle on their 1970 LP Cruel Sister which has subsequently been picked up by many folk singers as being traditional. Both the melody and the refrain come from the ballad known as "Riddles Wisely Expounded" (Child #1).)
Here, the refrain is syntactically independent of the narrative poem in the song, and has no obvious relationship to its subject, and indeed little inherent meaning at all. The device can also convey material which relates to the subject of the poem. Such a refrain is found in Dante Gabriel Rossetti's "Troy Town":
Phrases of apparent nonsense in refrains (Lay the bent to the bonny broom?), and solfege syllables such as fa la la, familiar from the Christmas carol "Deck the Halls with Boughs of Holly", have given rise to much speculation. Some believe that the traditional refrain Hob a derry down O encountered in some English folksongs is in fact an ancient Celtic phrase meaning "dance around the oak tree." These suggestions remain controversial.
There are two distinct uses of the word "chorus." In the thirty-two bar song form that was most common in the earlier twentieth-century popular music (especially the Tin Pan Alley tradition), "chorus" referred to the entire main section of the song (which was in a thirty-two bar AABA form). Beginning in the rock music of the 1950s, another form became more common in commercial pop music, which was based in an open-ended cycle of verses instead of a fixed 32-bar form. In this form (which is more common than thirty-two bar form in later-twentieth century pop music), "choruses" with fixed lyrics are alternated with "verses" in which the lyrics are different with each repetition. In this use of the word, chorus contrasts with the verse, which usually has a sense of leading up to the chorus. "Many popular songs, particularly from early in this century, are in a verse and a chorus (refrain) form. Most popular songs from the middle of the century consist only of a chorus."
While the terms 'refrain' and 'chorus' often are used synonymously, it has been suggested to use 'refrain' exclusively for a recurring line of identical text and melody which is part of a formal section--an A section in an AABA form (as in I Got Rhythm: "...who could ask for anything more?") or a verse (as in Blowing in the Wind: "...the answer my friend is blowing in the wind")--whereas 'chorus' shall refer to a discrete form part (as in Yellow Submarine: "We all live in a..."). According to the musicologists Ralf von Appen and Markus Frei-Hauenschild,
In German, the term, "Refrain," is used synonymously with "chorus" when referring to a chorus within the verse/chorus form. At least one English-language author, Richard Middleton, uses the term in the same way. In English usage, however, the term, »refrain« typically refers to what in German is more precisely called the »Refrainzeile« (refrain line): a lyric at the beginning or end of a section that is repeated in every iteration. In this usage, the refrain does not constitute a discrete, independent section within the form. 
Many Tin-Pan Alley songs using thirty-two bar form are central to the traditional jazz repertoire. In jazz arrangements the word "chorus" refers to the same unit of music as in the Tin Pan Alley tradition, but unlike the Tin Pan Alley tradition a single song can have more than one chorus. Von Appen and Frei-Hauenschild explain, "The term, 'chorus' can also refer to a single iteration of the entire 32 bars of the AABA form, especially among jazz musicians, who improvise over multiple repetitions of such choruses." 
In jazz, an arranger's chorus is where the arranger uses particularly elaborate techniques to exhibit his or her skill and to impress the listener. This may include use of counterpoint, reharmonization, tone color, or any other arranging device. The arranger's chorus is generally not the first or the last chorus of a jazz performance.
In jazz, a shout chorus (occasionally: out chorus) is usually the last chorus of a big band arrangement, and is characterized by being the most energetic, lively, and exciting and by containing the musical climax of the piece. A shout chorus characteristically employs extreme ranges, loud dynamics, and a re-arrangement of melodic motives into short, accented riffs. Shout choruses often feature tutti or concerted writing, but may also use contrapuntal writing or call and response between the brass and saxophones, or between the ensemble and the drummer. Additionally, brass players frequently use extended techniques such as falls, doits, turns, and shakes to add excitement.