Tom Hanks has been asked in various interviews what this movie is about [for example on the Castaway DVD]. He has said it's a fairytale of sorts, but he doesn't know what it's about. If the main actor can't say what it's about, far be it from me to try. It follows a curiously charismatic idiot savant through the 1960,70s and 80s and major events of that generation -- those events for which everyone knows where they were when it happened (like when Kennedy was shot). Beyond that I surely can't say what it's about. I will say the ensemble cast is superb. Hanks definitely deserved the Oscar for this performance, and I would love to see the outtakes for some of the 'how did he manage that' scenes -- like the one in Jennie's dorm room. I also liked the child actors. The boy who plays young Gump is perfect. That was his natural way of speaking, according to Zemekis (the director), and Hanks copied it to get the adult Gump's sing-song way of speaking. The DVD extras have some casting takes where the actors were reading lines in a studio, and Hanks doesn't have the Gump speech pattern yet in those.
 The movie is based on the novel, Forrest Gump. That "box of chocolates" tag-line is a rather liberal revision of the line in the book, "Being an idiot is no box of chocolates". I think we can safely assume the movie is not a faithful rendition of the novel.