When you pay someone to provide a service to your company, you are often required to send him/her a Form 1099 at the end of the year. The Form 1099 shows the amount paid, and his name, address, and tax i.d. number, which can be a social security number or a business i.d. number.
I recommend getting this information, in writing,
before you cut the first check. You don't want to
be asking for this information at the last minute
before the Form 1099 deadline.
The IRS recommends having your payee fill out and
sign a Form W-9, but as long as he gives you the
information in writing, with a signature, you are okay.