R+L=J, A Love Story: Rhaegar + Lyanna = Jon * A Song of Ice and Fire * Game of Thrones

Rhaegar and Lyanna, the Tourney at Harrenhal, Robert's Rebellion, baby Jon snow, the Prince That Was Promised (one part anyway) - what's this R+L=J thing all about? Why did they do what they did, and what was it like for them?