This is a piece of organ music believed to be composed by Bach sometime in the middle of the first decade of the 18th century. It is one of the most famous works in the organ repertoire.
The attribution of the piece to Bach has been challenged since the 1980s by a number of scholars for stylistic reasons. However as no definite name has cropped up as the piece's composer, it is still usually listed under Bach's name.
It is believed that this piece was almost certainly not conceived for the organ and was probably originally written for the violin.
The conductor Leoplod Stokowski made an effective orchestral arrangement and is largely responsible for the popularity of this work. His 1927 78rpm disc was an international best-seller which introduced the music to many record collectors and in 1940 his transcription was famously featured in Walt Disney's Fantasia cartoon. Since then the piece has been much used in a variety of popular media often in frightening scenes and those not au fait with classical compositions still recognize it as "that scary piece of organ music." Somehow the tune has become associated with Phantom Of The Opera, although it has nothing to do with it.
On two occasions, rocked up versions of this piece have been Pop music hits. The English classical/rock fusion band Sky reached #5 in the UK charts with their 1980 arrangement and in 1995 violinist Vanessa-Mae peaked at #16 in the UK with her pop version, which returned the piece to its original violin roots.