A Tax Identification Number or "ITIN" is a number that is 9 digits that is issued to individuals by the IRS who have to have a number to be identified by but are not eligible to receive a social security number or an Employer ID Number.
A SSN or Social Security number is issued to a person for identifying purposes as well as for tax reporting.
The ITIN number cannot be used for identification purposes but the SSN can.
If you have an SSN number then you won't need or be able to get the ITIN number.