Mid Bass usually only extends to arround 250Hz. It generally picks up the space between the subwoofers and the midrange where they do not operate as efficiently.
With Midbass, you notice more of a kick from the speakers.
Midrange, you notice more vocals, guitars, pianos. etc....
Both are necessary for optimum sound quality. Today, many manufacturers are combining the two together into one package- mostly in component systems. However, there are still distinct advantages in running them separately when you have the space to do so. A friend of mine did an install on his Buick Skylark a while back that consisted of an 8" midbass, 4" midrange, and 1" tweeter, all placed in the kick panel area. Needless to say, the front stage was incredibly powerful and very impressive SQ wise.
take it easy,
-zane