What is the Barcode on an envelope called for USPS?

[WannaFly]

Example:
http://www.openssh.org/ssh-dispute/envelope.jpg

In the bottom right of the envelope theres a "barcode" - a bunch of lines that USPS uses for SOMETHING. What is that called? I'm making custom paper that has that on it and I need to know where I can find more information on it. Thanks.

 

[kranky]

Postnet.

 

[WannaFly]

kranky: thanks that helped a ton, I found some great information - probably what I needed, thanks.

 

[kranky]

Glad it helped.