People! A sousaphone is a tuba. It was named after John Philip Sousa, who ordered it as a way to have a tuba that one could hold and carry easily. Sousa is known for writing pretty much all of the major march-type pieces that America holds dear.
He also wrote this fine work, which features the tuba nicely, and you'll hear it if you have something configured on your system to play OGG files, such as Quicktime:
http://cellar.org/2009/liberty_bell_march.ogg