If you didn't purchase it from the iTunes store, probably because the iPhone only supports h.264 encoding. You will have to convert the video to that format or get an app that will play the format that it is in. To convert, first import it into iTunes if it isn't already. Then right click on it and choose the convert option.
This site contains user submitted content, comments and opinions and is for informational purposes only.
Apple disclaims any and all liability for the acts, omissions and conduct of any third parties in connection with or related to your use of the site.