Most people who are born in or authorized to work in the United States receive what's called a Social Security number (SSN). This allows amounts to be deducted from their salary over their working life, and makes them eligible for benefits upon retirement or becoming disabled.
If you are in the U.S. in a status that does not allow you to receive a Social Security number, however—perhaps because you are an undocumented immigrant, a nonimmigrant with a visa that doesn't allow work, or a spouse of a U.S. citizen, green card holder, or visa holder in the United States and you're not allowed to work during your time here—you cannot become part of this system.
Something called an Individual Tax Identification Number (ITIN), however, issued by the Internal Revenue Service (IRS), offers some similar benefits when it comes to showing that compliance your U.S. tax obligations.
The main purpose of the nine-digit ITIN is that it can be used in place of an SSN for purposes of filing income taxes or claiming various tax credits (such as the child care credit for a U.S. citizen or lawful resident child).
It will not, however, fool an employer into thinking you have a right to work in the United States. The employer can easily check a government database to see whether you have a valid SSN. The ITIN will only allow an employer who is willing to take a chance on (illegally) hiring an undocumented immigrant to withhold taxes on your behalf.
The ITIN also comes in handy for things like opening an interest-bearing bank account, applying for a driver's license (the laws in some states allow this to people with ITINs), and creating a track record of having lived in the U.S. (which is occasionally useful for gaining forms of temporary or permanent immigration status, in the nature of an amnesty).
Keep in mind, however, that the ITIN does not indicate or confirm legal status in the U.S., nor grant any right to legally work here.
To apply for an ITIN, you will need to download Form W-7, Application for IRS Individual Taxpayer Identification Number, from the IRS website.
To this, you will need to attach a completed federal income tax return. (The purpose of this is to show that you need the ITIN for tax-filing purposes.) This can be either an original or a copy of a return you filed in the past (in which case you'll need to write "COPY" at the top.)
Then you will need to gather original documentation or certified copies verifying your identity and foreign status. The only documents the IRS will accept are the following, which must be current and show an expiration date and your name and photograph:
You can apply either by mail or in person at a local IRS office. If successful, you will receive a letter containing your number. You will not receive an actual card; this was a move to avoid confusion with SSNs and their accompanying status to work.
Once you have an ITIN, you will need to use it regularly (namely by entering it onto a 1040 tax return, or being claimed as a dependent on someone else's tax return).
If you don't use it for three years in a row, you will need to revalidate it—that is, reapply. For information on how to do that, go to the Individual Taxpayer Identification Number (ITIN) page of the IRS website.