I far as I understand. PIN is a way for Blackberry users to communicate with one another Blackberry to Blackber, kinda like text messaging. Each Blackberry has its own unique PIN, if you know someone elses you can PIN to them and vice versa.
If you use the search on this site you can find out much more about it, or anything else for that matter. These forums are wealth of information, with a lot of helpful posters as well