The W-9 form provides the key data clients need if you're an independent contractor. Just be sure you're not really an employee who should fill out a W-4.

If you want to be paid, refusing to hand over a Form W-9 can be a mistake. You may face backup tax withholding at 24% if you fail to sign and provide a Form W-9.

If you want to be paid, refusing to hand over a W-9 may not make sense. The IRS says that anytime a payor thinks they may have to report a payment on an IRS Form 1099, they should ask for a Form W-9.

If you haven't looked at them yet, all those numbers and letters – W-2, W-9, 1099 – can easily overwhelm you. Some forms are pre-populated and designed for you to pass on to the IRS.