The short version is simple. Google Lit Trips are free downloadable files that mark the journeys of characters from famous literature on the surface of Google Earth. At each location along the journey there are placemarks with pop-up windows containing a variety of resources including relevant media, thought provoking discussion starters, and links to supplementary information about "real world" references made in that particular portion of the story.
For media folks like me, the cool, interactive graphical program that is Google Earth has always had a Holy Grail-inserting moving media so it plays right over the terrain, as still images will do. We've gotten a lot closer with two new features, one with mixed blessings. First, Google Earth now has a YouTube layer.