User Guide - This section of the documentation provides tutorials, videos and tips on how to accomplish certain tasks using a wide range of features within the application.
Reference Guide - This section of the documentation focuses on breaking the application down into all of its smaller parts. Things like what something is called, where you can find it and what each option does or means. It serves as the foundation for which the tutorials, videos and tips in the User Guide section can be built on top of and refer back to for more specific information.
Below are links to the product pages, on the DAZ 3D site: