For an individual, a Tax ID is his/her Social Security Number. It is a 9 digit number that identifies you as you. It's format is XXX-XX-XXXX, and no two people have the same number.
For a business, the Tax ID number is called an Employer Identification Number (EIN), and it's format is XX-XXXXXXX (also 9 digits).
For a business, the Tax ID number is called an Employer Identification Number (EIN), and it's format is XX-XXXXXXX (also 9 digits).