[Pontiac]

A Taxpayer Identification Number (TIN) is an identification number used by the Internal Revenue Service (IRS) in the administration of tax laws. It is issued either by the Social Security Administration (SSA) or by the IRS. A Social Security number (SSN) is issued by the SSA whereas all other TINs are issued by the IRS.

From the IRS website.

Typically the only time the IRS issues TINs is to aliens with Green Cards.

[CurlyDave]

...Typically the only time the IRS issues TINs is to aliens with Green Cards...

Not really true. I am owner of a small business and we have a TIN, issued by the IRS and different than a SS number -- different format even.

It is very convenient for times when some form wants a SS number and I don't want to give it to them. I just write in my TIN. Supposedly the IRS is the only entity that can correlate my TIN with my SS.