Possibly the most famous piece of organ music ever written, Johann Sebastian Bach composed Toccata and Fugue in D minor at some point in the first half of the 18th century. A toccata is a piece featuring lots of short, fast notes, while a fugue is a polyphonic structure where the main melody or ‘subject’ comes in each part in turn:
The fugue here starts around 4:30.
You can see in the video the organist using four manuals (keyboards) and the pedal board (which is just like another keyboard for your feet). Playing the organ is a real feat of co-ordination! All of the switches and buttons you can see on the sides of the manuals are so that you can choose the different sounds for each manual.
The piece has been orchestrated by Leopold Stokowski and appears in the legendary 1940 Disney film Fantasia: