Who is Sean Parker?

Silicon Valley billionaire Sean Parker announced today that he’ll give $250 million to study cancer treatments. The cash infusion is the largest ever toward cancer immunotherapy. Parker, of Napster and Facebook fame, is known as a partier and an eccentric. For example, he drew headlines with his elaborate “Lord of the Rings”-themed wedding in the California Redwoods. Parker is intent on making his mark. Who is he?