The function of identification is to map a known quantity to an unknown entity so as to make it known. The known quantity is called the identifier (or Id) and the unknown entity is what needs identification. A basic requirement for identification is that the Id be unique. Ids may be scoped, that is, they are unique only within a particular scope. Ids may also be built out of a collection of quantities such that they are unique on the collective.
To retrieve the data stored about an individual with the Internal Revenue Service (IRS) of the United States, the IRS needs to be supplied with the individual's Social Security number (SSN). The SSN, then, "identifies" a unique individual to the IRS. No other living person has that SSN and that individual is assumed to not have more than one SSN. Other data that the individual's SSN identifies with the IRS would be such things as his or her name, birthdate, and current employer.