Source: http://ww2.kcd.org/CA_Students/MarcusFlewellen/musicalinstruments/brass/brass.htm
The sousaphone might be regarded as a marching-band-friendly tuba (compare the images of the tuba and sousaphone). The traditional tuba is too cumbersome to carry around while marching. In the 1890s the American bandmaster, John Philip Sousa, requested a redesign so that the instrument would wrap around the body of the musician, and making it easier to carry and handle. Carrying the instrument around might not have been part of his original design plan, but the instrument became popular with marching bands.
Other variations different from the tuba include the orienation of the bell, and the diameter of the bell. Apart form these differences, the instruments are very similar in length and range.
Source: http://ww2.kcd.org/CA_Students/MarcusFlewellen/musicalinstruments/brass/brass.htm
Source: http://wn.com/tuba
http://www.youtube.com/watch?v=EsLDe5fYDrs