I often see (and use) the terms Design Research and User Research interchangeably, although I believe they are overlapping, but different.
-Design Research: This is the broader term. It can refer to the study of design itself (meta-research, if you will). It can also refer to research done in the context of design, but that can include research on materials, processes, users, etc. For example, comparing different types of display technologies, from the perspectives of cost, power consumption or legibility are all cases of design research.
-User Research: The study of the users of a product or system (or potential users for a new one), that can include ethnography, usability testing, etc to inform the appropriate design of that product.
So according to my definitions, User Research is really a type of Design Research, and not all Design Research is User Research.