A W-9 form is an IRS form that is also called a "Request for Taxpayer Identification Number and Certification." The W-9 form is used by any individual defined under law as a "U.S. person" or "resident alien" to verify his or her taxpayer identification number (TIN) with an employer or other entity.

Any entity with which you interact that is required to file any sort of information return with the IRS that includes interactions with you as an individual must have your TIN. The entity, whether it's an employer, a mortgage company, a real estate company, and so on, must have your correct Taxpayer Identification Number in order to report you accurately to the IRS. Even any company that issues dividends to shareholders must have your Taxpayer Identification Number when filing an information report with the IRS.

Don't forget your rights, however. Even though you are required by law as an employee to give certain information to your employer, your privacy is still protect under similar laws. This means that if your employer uses your personal information, such as your Social Security Number, for any unlawful use, or shares it with anyone else, then they can be charged in both a civil and a criminal court of law.

Once again, if you are defined under law as a "foreign person" rather than a "U.S. person" when you are filing your federal taxes, then you have to use the correct W-8 form rather than the W-9 form.

The W-9 is the form used by the IRS that essentially requires the taxpayer to give his or her Social Security number, his or her employer identification number, or any other identifier to anyone that pays them money or to whom they pay money, such as a mortgage company. This is so that any interest paid, any royalties paid, any dividends paid, or any other payment that an entity or an individual makes to you as an individual can be reported accurately and properly to the IRS. If you do not fill out a W-9 form and give the information to your employer or the dividend paying company in which you own stock, then some payment must be withheld by the payor.

Let's look at some examples. If you work as a part time referee, whether you referee Little League games or high school sports, you have to fill out a W-9 before you begin work for those few hours. The school or whoever pays you is going to have to report the money that they pay you to the IRS, so they have to be able to identify you as the recipient of payment with your Taxpayer Identification Number. If you are an independent contractor-for example, if you are a graphic designer, and you are hired to design and complete a website or marketing material for a company, before they can pay you and before you even begin work, you need to fill out a W-9.

If you fill out a W-9, and you are an independent contractor, then you will have to file an IRS form 1099 to report the income that you make that doesn't have taxes already taken out of it. This is income that is not paid through a particular company that takes out Social Security and other taxes, and then issues you a W-2 form.