An SSN (Social Security Number) is a nine-digit number issued by the Social Security Administration to all legal aliens allowed to work in the US.
An ITIN (Individual Taxpayer Identification Number) is a nine-digit number issued by the US Internal Revenue Service (IRS) to individuals who are required for US federal tax purposes to have a US taxpayer identification number but who do not have and are not eligible to get a Social Security Number (SSN).