An ITIN, or Individual Taxpayer Identification Number, is for taxpayers who don't qualify for a Social Security Number (SSN). It's a 9-digit number that looks like an SSN except it always begins with the number 9.
ITINs are used exclusively for filing tax returns. They don't entitle the taxpayer to Social Security or other benefits.
To apply for an ITIN, complete IRS Form W-7, IRS Application for Individual Taxpayer Identification Number. The Form W-7 requires documentation substantiating foreign/alien status and true identity for each individual.
You can mail the W-7 and documentation to the address in the Form W-7 Instructions, present it at a local IRS office, or use an Acceptance Agent, which are places like colleges, financial institutions, or accounting firms that are authorized by the IRS to assist applicants in obtaining ITINs. They'll review the W-7 and documentation before sending it to the IRS.
Form W-7(SP), Solicitud de Número de Identificación Personal del Contribuyente del Servicio de Impuestos Internos is available for Spanish speakers.