My one coworker had to describe abstractly how she would implement a hash map to the interviewer, and I suspect you'll be asked similar things. Other than that, I recommend basic data structures, big-O notation, maybe some basic set theory (if you're doing any sort of database work). If you've taken a discrete math or algorithms course, then expect some of the more fundamental stuff from that.
What year in school are you? I got my first summer internship between sophomore and junior years, and looking back, I really didn't know anything useful, yet they still took me and I'm still working for them now. Just make sure at the very least that you appear (or are) eager to learn and solve problems, and be personable and polite. They're looking for potential at this point, not necessarily what you know.