PCMI is a 3 week program; each day from about 830 to 1040 we have what can best be described as a math class. But it's unlike any math class most people have ever had.
Each day starts with its own problem set designed by the class' organizers, folks from the Education Development Center and Harvey Mudd College. The problem set is well structured, beginning with a simple idea or concept and then continually developing in both depth and breadth, although this may be obvious only several days later. The questions are also in categories: Important (things you'll need to know for upcoming days), Neat and Tough (can be really tough! Clay Prize tough!) -- we aim to get through at least the important stuff in our morning together.
The classroom is composed of 12 tables of 5-6 people each (we do have guests from the other programs) and as a table we tend to worth through things together; there's a table sandbox monitor who is there to ensure that the teachers exercise all those collaborative skills they try to encourage with their students. Not only that, but we never tell people ideas, we create a situation in which they can they discover it themselves. This is not easy and like any skill takes practice and continual reinforcement. It is at the heart of the whole morning class (indeed, of PCMI) and the mathematics could almost be the motivation for appreciating this whole process. It's why I call them "organizers" above and not teachers -- it's not instruction as you know it.
The math is very accessible and very deep - low threshold, high ceiling - and it is too easy to look at it only superficially. Teachers will occasionally race through the questions to get them done (remind you of any students?) and will miss out on the complexity of the mathematics. I remember my first year doing the same thing.
As one of the participants said "I've taken courses in number theory but never understood prime numbers until now." This has been true for every topic I've encountered at PCMI -- teachers seldom get the chance to think deeply of simple things that Al Cuoco of EDC, and one of the course's authors, encourages.
If you visit PCMI @ the Math Forum you can click on Class Notes to read over the problem sets from previous years. Or, to get a very insufficient glimpse of the questions, the MAA has a book of Al's work Mathematical Connections that includes material we've looked at during PCMI. It's condensed (remember, we get three weeks) and doesn't have the same level of personalization that our questions set have -- the authors adapt the problem sets from day-to-day to build off of our ideas, suggestions, questions & comments.