Here is my standard, canned response that I cut and paste to answer that question:
PITH - Pen In The Hat is an exchange of pens between members. Although the focus is on an exchange of finished pens, the exchanges occasionally include pen kits, pen blanks, and other "goodies". More than that though, they facilitate making a stronger connection to another enthusiast and IAP member. The pens I have received in these exchanges in the past hold a very special meaning to me personally.
Traditionally PITH events have been held as part of the IAP Anniversary BASH celebrations and Summer Extravaganzas. These typically involve some basic rules set by the organizer of the event. Participants sign up to commit their willingness to partake in the event. At a designated time, the list is arranged (usually randomly) to pair individuals together. The list is posted and the participants exchange greetings, addresses, and other information needed to complete their swap. This information is typically exchanged using the IAP PM (Private Message) system, (For those that haven't experienced it yet, the PM system is accessed by clicking on the "Start conversation" link at the bottom of forum posts).
In addition to these larger, PITH events that are connected to an anniversary or other celebration, a single member can instigate a PITH exchange at any time by posting their desire, rules, and expectations in the "PITH (Pen In The Hat - Pen Swaps) forum. Just as in the larger PITH events, the participants exchange information and the organizer orchestrates the the swap. After they are completed, pictures of pen swaps are usually posted in the forums for all to share.
Also, it can even just be a two person event. At any time an individual may post their desire to do a PITH exchange with just one other individual along with their rules and expectations. I have done these occasionally with new or relatively new members that may be apprehensive about their beginning level skills. I've never received a PITH pen that I don't treasure.
They are a lot of fun. I hope you will decide to participate!