At a cajun/zydeco dance weekend, we were taught that the difference between cajun and zydeco was about six inches. Cajun dances maintain a touch of propriety. Zydeco is ABOUT touch!
There is another sub genre too- Fay do do . Just like Zydeco, it takes its name from a single song. "Fais dodo" is French baby talk for "go to sleep", and the song became so popular that it became the name of the style.
Cheers
Dave