The sousaphone *is* a tuba, as I understand it -- but not every tuba is a sousaphone.

Marching band maven John Philip Sousa redesigned the standard tuba for maximum portability by wrapping the horn in circles around the player's body (instead of winding it up in a wad to be held in front of the body). Same length of horn, just coiled up in a different configuration.

In the US if not elsewhere, the sousaphone is probably more familiar to more people than the original tuba because it is more visible as a marching-band instrument -- but the name is not nearly so familiar. The masses generally recognize the sausaphone as a "tuba" (if not as "the" tuba).