A motorhome tends to be larger and is more modular in its layout. The interior will be separated out to create living spaces with permanent fixtures. These will typically include a kitchen area, living area and sleeping areas. Motorhomes will also benefit from a toilet and shower whereas a campervan will not.
Campervans on the other hand are smaller and will only have basic amenities such as a sleeping area and cooking area.
A motorhome is clearly defined by the DVLA as well and requires separate insurance. It is possible to cover some campervans under a van or car policy though depending on the vehicles and its facilities.