Woah, there p_dog. He rolled his eyes. No need to get serious. And April 2007 isn't older than 95% of the forum. We've been getting a pretty constant stream since...September of 2006. That puts you at about halfway between now and the beginning. Now please, be civil. Escalating the situation only makes you look like a jerk. Dragoon's response was unnecessary, but whose was worse? I'll let you judge.
As for the question, PM stands for Private Message. To send a PM, you can click on a person's name, and then select PM in their profile (it will send them a Private Message). You can view your PMs, or compose new ones, by clicking Inbox in the top right, just after your name and Sign Out.
I like pi.