If you're an employer, or even just making a payment to a vendor, you've got some responsibilities to Uncle Sam. Anytime you hire someone, or engage in a business relationship with an independent contractor, you must generate an IRS form W-9, Request for Taxpayer Identification Number and Certification.
You need to have your employee or contractor fill it out and sign it so you know their Social Security number or taxpayer identification number of U.S. persons attempting to work in the United States. You don't need to send it in to the IRS. Keep it for your records, so you'll have it when you generate your W-2s and 1099s.