An Excerpt from the Preface of "Sufi Music of India and Pakistan" 
by Regula Burckhardt Qureshi

Qawwali considered as music is a group song performed by qawwals, professional musicians who perform in groups led by one or two solo singers. Qawwals present mystical poetry in Farsi, Hindi and Urdu in a fluid style of alternating solo and group passages characterized by repetition and improvisation. The vigorous drum accompaniment on the barrel-shaped dholak is reinforced by handclapping while the small portable harmonium, usually in the hands of the lead singer, underscores the song melody. A Qawwali song normally begins with an Instrumental Prelude on the harmonium; then an Introductory Verse is sung as a solo recitative without drums, leading directly into the song proper: a mystical poem set to a strophic tune and performed by the entire group of Qawwals.

Qawwali considered as an occasion is a gathering for the purpose of realizing ideals of Islamic mysticism through the ritual of listening to music', or sama'. By enhancing the message of mystical poetry, and by providing a powerful rhythm suggesting the ceaseless repetition of God's name (zikr), the music of Qawwali has a religious function: to arouse mystical love, even divine ecstasy, the core experience of Sufism. The Qawwali assembly is held under the guidance of a spiritual leader, and is attended by Sufi devotees though it is usually open to all comers. In listening to the songs, devotees respond individually and spontaneously, but in accordance with social and religious convention, expressing states of mystical love. The musicians, for their part, structure their performance to activate and reinforce these emotions, adapting it to the changing needs of their listeners.

To the Sufi participant, Qawwali is 'a method of worship' and 'a means of spiritual advancement'; it is also 'a feast for the soul'. To the performer it is mainly a musical genre 'with its distinct character for worship'. To the observer, finally, Qawwali is above all music performed very obviously with continual reference to its context; it is 'music in context' par excellence.