Sample Social Security Number Card
Social security number card is generally the proof of authorization to work in the United States (unless it specifically states that 'Not valid for work without INS/USCIS authorization').
If you are on non-immigrant visa (such as H1, L1 etc.), you may get the social security card that says 'Valid for work only with INS/USCIS authorization').
If it does not mention anything like that, you are not restricted to work for any particular employer. Such card would be available to the US citizen, permanent residents (green card holders) and the persons with Employment Authorization Card (EAD).
Social security numbers are always in the format of nnn-nn-nnn.