This is an excellent question. I know that a lot of people argue about PBL and discuss it on SDN without really understanding what PBL actually is. For the uninitiated, PBL stands for problem-based learning. It is a student-centered way of learning, in contrast to lectures, which are faculty-centered. The two are complete opposites. Confused? Read on.
The typical class that you remember from college is a faculty-centered lecture. What this means is that you have a professor or instructor get up in front of a room (typically an auditorium if you went to a large state university like a lot of us did!), and talk to you for an hour or two. The instructor decides what to discuss, tells you what to read beforehand, and chooses what homework you will be assigned. The instructor also makes the tests and is the sole person who evaluates you. You, the student, sit passively and take notes on every word that the instructor says. About the most involved you get is when you ask the professor a question about something he said, or if the professor gives you one of those clickers so that you can respond to the polls on his Power Point.
PBL is completely at the opposite side of the spectrum. The faculty tutor who is assigned to our group speaks very little, and sometimes not at all. In our sessions, it is the students who are responsible for leading the group and setting the agenda. We have one student serve as the leader each time, and in my group, we swap leaders after each case (once per week). All students in the group are expected to participate equally, which means that we are all teachers and we are all students. As a group, we work through a medical case that has been assigned to us. Based on this case, we hypothesize about what problem the patient could have, and we make a list of what we know and what we don't know and would like to know. From the second list, we come up with a group of learning objectives. Each student takes a learning objective, reads about it, and prepares a brief (5-10 minute) presentation about it to share with the rest of the group during our next session. In a typical session, all of us present our learning objectives, and we then continue on with the case. As we add new information, we modify our hypotheses, come up with new learning objectives on things that we still don't know, and try to integrate what we are learning with what we already know. Periodically, we all evaluate ourselves and each other. Our faculty tutor also evaluates us, but his evaluation is one of many instead of the sole evaluation.
I will warn you up front that PBL does have a higher learning curve, and it takes some getting used to if you've never done it before, which most of us hadn't before coming to medical school. But I definitely feel that it is worth it. I have now had the opportunity to experience both PBL and lecture-style curricula, and I can tell you that PBL is WAY more fun than sitting through lectures all day. It is also a lot more work to set up a good PBL session than it is to just attend a lecture. But I am constantly amazed by how much I learn not only from preparing my own learning objectives, but also from the ones prepared by my classmates. PBL forces you to work with your classmates and adjust to everyone's unique background and learning style. It is impossible to be selfish in PBL, because you are responsible for everyone's learning and not just your own.
If you're interested in reading more about PBL, here are two great websites that talk about it quite a bit: http://www.mcli.dist.maricopa.edu/pbl/info.html and http://www.pbli.org/pbl/pbl.htm PBL is in use at many other medical schools besides ours, but CCLCM is one of the few schools that were designed from their very beginning to use PBL instead of lectures. I think that this is one of the reasons why PBL is so well-integrated into our curriculum instead of seeming like it was tacked on as an afterthought. If you think that you'd enjoy a PBL curriculum, you should definitely apply to schools and programs that use it. Some people might try to scare you and tell you that PBL won't teach you everything you need to know or won't prepare you for the medical boards, but the scientific literature doesn't support this conclusion. Actually, PBL-based curricula seem to prepare people equally well versus standard curricula, and again, the PBL is a lot more active and fun to do compared with sitting through yet another mind-numbing lecture. Here is one example of a paper on this issue.