It probably depends very much on the school, as well as on the subject you want to study. but in general, they are more interested in how you think and what kind of person you are, and less in your grades from your undergraduate studies.
They will ask you general questions, like why you want to study this subject, why at this particular school, and other similar questions. This is more designed to see how serious you are and how motivated you are.
You should mention any work experience that you have that is relevant to the application (if it isn't already on your application), and be prepared to answer questions about it. They might also ask you general questions about the subject, or about things you studied during your undergrad studies.
Try and give clear and concise answers, and if they ask you something you don't know be truthful and say that you don't know (it looks better than guessing wildly)
Good luck