Commas set off a word, phrase, or clause that is in apposition to (that is, equivalent to) a preceding or following noun and that is nonrestrictive.

• It sat nursing its front paw, the injured one.
  
• Aleister Crowley, Britain's most infamous satanist, is the subject of a remarkable new biography.
  
• A cherished landmark in the city, the Hotel Sandburg has managed once again to escape the wrecking ball.
  
• The committee cochairs were a lawyer, John Larson, and an educator, Mary Conway.
  

Restrictive appositives are not set off by commas.

• He next had a walk-on role in the movie The Firm.
  
• Longfellow's poem Evangeline was a favorite of my grandmother's.
  
• The committee cochairs were the lawyer John Larson and the educator Mary Conway.
  
• Lord Castlereagh was that strange anomaly[,] a Labor-voting peer.

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