I am assuming you are asking about freestyle and pre-choreographed classes? (Both can be structured).
Freestyle classes are usually classes where the instructor has control over preparing the class–including any choreography and the exercises. It allows you to be creative while taking different skill levels into consideration.
Pre-choreographed classes are prepared classes where you follow a specific routine, format, and for a specified amount of time. Sometimes you must also use the music that comes with the program–and you also pay for the updates. (An example of a pre-choreographed program would be Les Mills–although I don’t necessarily know if it follows everything I’ve mentioned).
There are pros and cons to both, and you may find others answer this in a different way (this is how I’ve always understood the difference). Personally, I’ve always done freestyle and love preparing my own classes and having control. Hope this helps!