Little is known of James Potter except that his son Harry, is very much like him. It has been told that he was one of the cleverest students at Hogwarts and, paired with Sirius Black, one of the two most trouble. Everyone who knew him (with perhaps the exception of Severus Snape) remembers him fondly.(And even Snape must admit, no matter how grudgingly that his life was saved by James)

He was able to master the animagus spell with his three friends, Remus, Sirius, and Peter, and took the shape of a stag, nicknamed Prongs. It is thought that perhaps Prongs lives on as the protective spirit in his son's Patronus spell. The spell does take the shape of a white stag and Harry once though he saw his father immediatly after casting it. The truth? Wishful thinking? It's hard to be sure.

Also known of James is that he married his school sweetheart, Lily. He later died in an effort to hold off Voldemort from her and their son. Why the Dark Lord had it in for the Potters is unknown. Perhaps he only had it in for Harry, as there is a belief that Lily was only killed because she got in the way, dying to save her son. James could have also been killed as merely am obstacle to the boy.

The only one who seems to know these answers is Headmaster Dumbledore and, for whatever reason, he's not talking. (I have a few theories, though...) At any rate, all we can do is hope for enlightenment, and wonder, as we must for all his comrades, when he'll be back.