It's a tax related form. It comes in a 4 page layout that uses the last three pages for the W-9 form instructions and the first page for the actual W-9 form that gets filled out by the person who gains the benefit of either a check or cash etc. payment or another means of gain.
There is also a spot for the Instructions for the Requester of Form W9.
It is formally called a "Request for Taxpayer Identification Number and Certification".
In a lot of cases, it is used as such:
Under penalties of perjury I certify that ... and it goes on to list what you are responsible for as far as perjury is concerned.
Essentially it means that you can not lie on the form or be deceptive although it is more specific. There is a section about backup withholding etc.
Sign Here - Signature of US person then there is a date section for the date signed.
This is usually the end of the section for most people who are doing a simple job of work but the form is used for several other reasons besides a simple job and so it has instructions as to filling out the form and how to put the proper information where.
There are special instructions about various things that could come up with the W-9 form, definition of a U.S. person, special rules for partnerships, and a lot more information and instructions on the following pages for more complex use of the form.
There is a section on Penalties! So be careful not to make a mistake on something that you have simply overlooked.
Also there is the W9S for students and borrowers.
There is the W8Ben for foreigners and the W8 Instructions are there as well.
There is a Spanish language W9Sp that is useful for Spanish speakign people.
Check the website www.PrintW9.com for a lot of tax information and these forms.