I've been looking at some online ad revenue providers. The US-based sites seem to want me to complete a W-8 form. What is the W-8 form and how should I go about completing it?
The IRS W-8BEN form (PDF link), titled "Certificate of Foreign Status of Beneficial Owner for United States Tax Withholding", certifies that you are not an American for tax purposes, so they won't withhold tax on your U.S. income.
You're also to use W-8BEN to identify your country of residence and corresponding tax identification number for tax treaty purposes. For instance, if you live in the U.K., which has a tax treaty with the U.S., your W-8BEN would indicate to the U.S. that you are not an American, and that your U.S. income is to be taxed by the U.K. instead of tax withheld in the U.S.
I've filled in that form a couple of times when opening stock trading accounts here in Canada. It was requested by the broker because in all likelihood I'd end up purchasing U.S.-listed stocks that would pay dividends. The W-8BEN is needed in order to reduce the U.S. withholding taxes on those dividends.
So I would say that the ad revenue provider is requesting you file one so they don't need to withhold full U.S. taxes on your ad revenue.
Detailed instructions on the W-8BEN form are also available from the IRS: Instruction W-8BEN (PDF link).
On the subject of ad revenue, Google also has some information about W8-BEN:
Why can't I submit a W8-BEN form as an individual?