You are asking a person for their "Taxpayer Identification number and/or Certification." There are details about how to go about this as the information that you will receive is important to keep safe.
Anyone who has a person's name address and social-security number has a lot of useful information about them for a thief or a scam artist to take advantage of the person.
For foreigners look at this form Instructions for the Requester of Forms W-8BEN, W-8ECI, W-8EXP, and W-8IMY
Now this form is informative for the user as it is a four page instructional publication and has a lot of details.
I think in general it is a simple device to understand in that you know why you are asking for the information already so feel free to read it just to know what the publishers are talking about.
Take a look at the iW9 form for use.
It is very informative and it covers titles such as: