Youtube and Google Video use the format flv, flash video, to stream the videos on their websites. If you download them using a firefox extension such as Video Downloader you might have problems playing them in your normal video player. One option would be to use a player that supports flv files such as the FLV Player. A better solution would be in my opinion to convert the flv files into avi or mpg which are widely supported and can be send to friends or burned on DVD easier.
