PIN aka Personal Identification Number service allows device to device direct messaging, like via BBM.
It also allows apps to pass on messages, like when you install a "free" app from BlackBerry world and then you start getting messages advertising so-and-sos app...
The PIN is a string of 8 digits in a format called Hexadecimal and is hard coded to your device. It makes it so your device is unique and identifiable among all other devices. This ID also lets app makers "key" an app to you, sorta like a key code when you buy some computer software, but stronger as it is locked to your hardware.
There is another service within a BlackBerry OS, called PIM services aka Personal Information Management.
This is the service within the core operating system on your device that manages contacts, calendar, messages, and the likes.