Social Security Number (SSN) is a unique number issued to US citizens, Permanent Residents, and to certain foreign nationals who meet the eligibility criteria. In the 1930s, the first Social Security Number was issued, and the federal government mandated that all legal residents and citizens of US must have a valid Social Security Card. The Social Security Administration (SSA) issues Social Security Numbers (SSN) to applicants who meet the criteria.

SSN is a nine-digit number divided into three parts. The first three numbers generally indicate the area numbers: the state of residence at the time the person applied for his or her first card; the middle two digits of the SSN, the group numbers which range from 01 through 99, merely serve to break the numbers into blocks of convenient size and the last four characters, the serial numbers ranging from 0001 through 9999 represent a straight numerical progression of assigned numbers.

Importance of SSN
A Social Security Number (SSN) issued by Social Security Administration, is a number issued to citizens, permanent residents, temporary (working) residents and foreign citizens in the United States. It is generally needed for employment purposes. If you plan to work, have an assistantship, or get a driver's license, you must have a Social Security Number.
Primary Purposes of SSN:
.Tracking working individuals for taxation purposes & yearly wages
.Trace entire credit history of any person
.Check eligibility for Social Security benefits
.Report wages to the government
.Payroll deductions for old age, survivors, and disability insurance
.Many institutions also use the SSN as the student ID number

Applying for SSN
I. U.S. Citizens
Following documents are required to apply for an SSN:
.Completed Form SS-5 (Application For A Social Security Card)
.Proof of U.S. citizenship: You may submit U.S. birth certificate, U.S. consular report of birth, U.S. passport, Certificate of Naturalization or Certificate of Citizenship
.Proof of Age: Your birth certificate or passport may be submitted to prove your age
.Proof of identity: Your identity must be current (not expired) and show your name, identifying information (date of birth or age) and preferably a recent photograph. Your U.S. driver's license; State-issued non-driver identification card or U.S. passport may be used to prove your identity. If you do not have one of these specific documents or you cannot get a replacement for one of them within 10 days, other documents that can be considered are:
      Employee ID card;
      School ID card;
      Health insurance card (not a Medicare card);
      U.S. military ID card; or
      Adoption decree.
Once you have collected all the required documents, submit your completed application and documents in person to your nearest local Social Security office. Persons of age 12 or older applying for an original Social Security number card must appear for an interview at a Social Security office. All documents must be either originals or copies certified by the issuing agency. Photocopies or notarized copies of documents are not accepted.

II. Non-citizens who have work authorization from DHS
Only non-citizens who have permission to work from the Department of Homeland Security (DHS) can apply for a Social Security number.
Following documents are required to apply for an SSN:
.Completed Form SS-5 (Application For A Social Security Card)
.Documents to prove your Immigration status & Work Eligibility: The I-94 (Arrival/Departure Record) issued to you when you arrived in the United States will be needed to prove your immigration status.
.If you are an F-1 or M-1 student, you will be required to show Form I-20 (Certificate of Eligibility for Nonimmigrant Student Status). If you are a J-1 or J-2 exchange visitor, you will be required to show your Form DS-2019 (Certificate of Eligibility for Exchange Visitor Status).
An F-1 student authorized to work in curricular practical training, must provide the Form I-20 with the employment page (page 3) completed and signed by a designated school official. Evidence of the employment, such as a recent pay slip or a letter from your employer will also be necessary. Your supervisor must sign and date the letter and it must describe the following:
       Your job
       Your employment start date
       The number of hours you are, or will be, working
       Your supervisor's name and telephone number
An F-1 or M-1 student authorized to work off campus, must provide the SSA with the Employment Authorization Document received from DHS. A J-1 student will be required to provide a letter from his/her sponsor. The letter should be on sponsor letterhead with an original signature that authorizes the employment.
Proof of age: Your birth certificate or passport may be submitted to prove your age
Proof of your identity: A current DHS document will be required by the Social Security Office. Acceptable documents include:
       Form I-551 (includes machine-readable immigrant visa with your unexpired foreign passport)
       I-94 with your unexpired foreign passport
       Work permit card from the Department of Homeland Security (I-766 or I-688B).
Once you have collected all the required documents, take your completed application and documents to your nearest local Social Security office. Persons of age 12 or older applying for an original Social Security number card must appear for an interview at a Social Security office. All documents must be either originals or copies certified by the issuing agency. Photocopies or notarized copies of documents are not accepted.

III. Non-citizens who have work authorization from DHS
Lawfully admitted non-citizens who are not authorized to work, can get many benefits and services without a Social Security number. SSN is not required to conduct business with a bank, register for school, apply for educational tests, obtain private health insurance, apply for school lunch programs or apply for subsidized housing.
Without permission to work, you may apply for a Social Security number only if:
      A federal law requires you to provide your Social Security number to get a particular benefit or service; or
      A state or local law requires you to provide your Social Security number to get general assistance benefits   that you already have qualified for.

Cost of SSN:
There is no charge for a Social Security number and card.

Types of Social Security Cards:
There are three types of Social Security cards which show your name and Social Security number.
.The first type of card is issued to:
       U.S. citizens; and
       People lawfully admitted to the United States on a permanent basis.
.The first type of card is issued to: 
       People lawfully admitted to the United States on a temporary basis who have DHS authorization to work.
.The third type of card is issued to:
      Foreigners who are lawfully admitted to the United States without work authorization from DHS, but with a  valid non-work reason for needing a Social Security number; or
      Foreigners who need a number because of a federal law requiring a Social Security number to get a benefit or service.

Processing Time
SSA needs to verify your documents with DHS before they assign a Social Security number to you. Most of the time, SSA can verify your documents quickly with DHS online; however, if your documents cannot be verified online, it may take DHS several weeks to respond to SSA’s request.

Can I start working without SSN?
SSA does not require you to have a Social Security number before you start work. However, the Internal Revenue Service requires employers to report wages using a Social Security number. While you wait for your Social Security number, your employer can use a letter from SSA stating that you applied for a number. Your employer may use your immigration documents as proof of your authorization to work in the United States.

Contacting Social Security
For more information about Social Security Administration, you may visit www.socialsecurity.gov