Art nude modelling is a variation of nude modelling. The model will likely be fully nude, with the art element of art nude often focusing much more on the playing or experimentation of light and shadows against the model or scene. Art nude is very rarely sexually explicit in nature and is much more focused around light/body studies.
Art nude may focus on the model's body or particular body parts. It often encompasses "bodyscapes" or "body studies" where the focus of the image will be one small area of a models body, such as a nipple.
A model looking to partake in art nude should be very comfortable working at a nude level and should have a good awareness of their own body and how different poses, movements and shapes will impact the lighting and how it falls over their body.