I'm also interviewing, but I generally have good success at job interviews, so here's my 0.02. I think the most important thing is to be polite and professional, and after that try to relax and be yourself. You're looking at spending 4-6 years with these people; both of you are going to get to know each other eventually, and it's best to find out if it's going to be a good fit now.
After that, I think it's fine if you don't understand some of the things they're researching. Be honest and say "X is really interesting to me, but I haven't had much exposure to it yet." Talk about what you have had exposure to and why you're interested in different things. Also, remember that if they're having you out for an interview, they're already interested. This is part review and sanity check (make sure you're well adjusted and didn't lie on your application) and part recruitment. They want you to leave there interested in them, and it's ok to ask about what they can do for you.
Out of curiousity, what school are you headed to?