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. Your SSN can be found on your Social Security Card.
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).