The Zapruder film shows without a doubt that there was more than one shooter. Oswald was (supposedly) shooting from the 6th floor of the Book Depository building, which was behind Kennedy, whose car was moving away from that location. The fatal headshot came from the front. The film shows that when he was hit, his head went back and to the left, which is totally impossible if he was shot from behind. Check out the movie JFK, by Oliver Stone. While I don't agreed with everything Stone portrays, it's still a really good movie and a good starting point if you want to learn more about the assassination.
Edited to include link showing Zapruder film (warning- VERY graphic).
http://video.google.com/videoplay?do...5770464697597#