I recommend a small notepad that fits in your pocket or purse/bag. Some people bring a full sized notepad in a portfolio, but then you have to carry it around with you. Odds are, the only thing you'd write down is a name (a suggestion of somebody to talk to) or email address.
Don't bring a backpack to the interviews. For some schools with large campuses, such as UCSD, you will do a lot of walking.
Don't worry too much about knowing what the interviewer does in any sort of detail. You're not interviewing to work in their lab at this stage and you can just ask them what they do. It's far more important to know what you do to be able to explain it to them, if they ask. They will probably only ask you specific questions about your work if they work in your field, but they might ask why you want to do a PhD, why you want to go to that school, why you want to research whatever area, etc.