I'd give them a quick overview that showed you knew what they were and could use one (if the GPS packed up
) and let them ask any more questions that they may feel appropriate to demonstrate an appropriate level of knowledge.
Also, you could simply ask them. This would be fair game I think because being lengthy when they're looking for an overview could be as bad as a two sentence answer when they wanted the secret plans to build their own.
One thing you could say though was a VOR will tell you where you are from it whereas an NDB will tell you where it is from you.
Like OE says, it depends what you are interviewing for.
Good luck