Here is the one and only definition of "fuselage"
https://en.wikipedia.org/wiki/Fuselage

The term "fuselage" kayak has been mal-adopted by some people and has no meaning as far as kayaks. They probably did this to add some glamour to the product they are trying to market and sell. "Fuselage this" and "fuselage that" has been perpetuated here for long enough that the term somehow was allowed to stick.

The proper way for that kind of boat is "skin-on-frame" or "skin".

Now, on the subject of the "difference"
The difference would be between boats built using the traditional techniques (bent ribs) vs. the boats built with frames cut out of plywood (plastic etc)

The actual differences would be as follows:
1. Frames allow better control over the boat shape and performance
2. Frames are usually easier to make
3. Ribs may be somewhat stronger on side impact, if certain types of wood is used.
4. How the boat feels and flexes is primarily dependent on spacing of frames/ribs, and number of chines. Since the boats made with ribs were typcially hard-chined (as few as 4 longitudinal elements + keel), they are likely to flex more than boats with mulitple chines.
5. Materials and the way boat is skinned is paramount as well. Boat skinned with 12oz nylon that has been skillfully skinned wet, will remain tight and feel "stiffer" than boat sloppily skinned with 8oz nylon.

There are a few more things, but these are main points.