Design workshops are workshops designed to allow Customers to visualise their ideal product/service and shape it in the way they want it to be. It would be more valuable if the workshop is facilitated by a Designer as they can help users visualise their ideas. The success of this type of workshops depend on the tools used to facilitate it.
Design Workshops works best following a Participatory Design process (when the same customers are involved in all the different phases of the project). That process help them understand more about the field of analysis, let them become more engaged with the project and leave them with more time to think about solutions.
If that approach is not possible it is anyway really important to structure the workshop in order to allow the customer to understand the context of research and feel comfortable to start envisioning it. During the workshop, customers dream about that product/service, build scenarios of use, and sometimes also quick prototypes.
Robert Chambers, Participatory Workshops: A Sourcebook of 21 Sets of Ideas and Activities (London: Earthscan Ltd, 2002)