Plutarch was a Greek historian, biographer, and philosopher. He lived circa 46-120 AD. After studying philosopy in Athens, Plutarch traveled the Mediterranean world, and gave lectures in Rome. As a historian, Plutarch is considered fairly reliable.
The most famous of Plutarch's works, and one that will be relied upon heavily as a source, is his Bioi Parelleloi (Parallel Lives). This is a series of biographies covering fifty lives of famous Greeks and Romans. Most of the biographies are paired with another biography, with Plutarch's comparisons attached. Because of his background as a philosopher, Plutarch's goal in his Lives is not to simply relate historic fact, but to portray the moral character of each individual.
Oxford Concise Companion to Classical Literature. ed. Howatson and Chilvers. Oxford. New York, 1993.