As you may know, earlier in Jan’ 08, Google AdSense had lowered down, Personal Identification Number (PIN) from $50 to $10, which is used to help verify publisher accounts and addresses for security purposes. Google send this PIN via standard mail when your account first reaches $10 in earnings. 

Google has created a demo video showing “what to look for in the mail, how to use the information?”. If you don't receive your first PIN, you can still request up to two more. Please note that aside from verifying your PIN, other holds may apply to your account.

Google, AdSense, PIN, Demo, Payments, Earnings, Internet Video, YouTube