hey geno007,
Congrats on your interviews. I had my columbia interview in early Jan and UMD in late Jan.
As the others have said about the columbia interview, it is very standard. My interviewer was Dr. Horowitz. He was very friendly, asking me about my file (LOR, transcript, essay), my extracurricular activities, dentistry (why, exposure, etc), and other questions along those lines. He also gave me a chance to say something about myself that was not asked, or that I wish for him to convey to the committee. Afterwards, we had the chance to interact with current students via a tour and lunch.
At UMD, it was very similar. I must say that UMD is very organized in their appraoch. The day started with an overview of the school and the curriculum. Afterwards, we were assigned a time for our interview. While we were waiting for our interview, we were brought down to the clinic to observe third and four year students at work. Actually, I had the chance to assist the student I was assigned to. It was a great chance to ask questions, and learn more about the school. We then had our interview (one faculty and one student interviewers).. again.. standard questions. They asked me why dentistry, questions regarding my essay, extracurricular activities, and anything I want them to know about me. Following the interview, the director of student services met one-on-one with us to chat and to get to know us better. Afterwards we went on a tour of the school, then to lunch with students. Then we were free to explore the school on our own.
As the others have said, be yourself.. be honest and relax... you should do fine. GOOD LUCK!
topher