Here are the usage instruction taken from the original site:
Use youtube-dl followed by a video URL. Example: youtube-dl "http://www.youtube.com/watch?v=foobar". The video will be saved to the file foobar.flv in that example. As YouTube.com videos are in Flash Video format, their extension should be flv. In Linux and other unices, video players using a recent version of ffmpeg can play them. That includes MPlayer, VLC, etc. Those two work under Windows and other platforms, but you could also get a specific FLV player of your taste.
Here's the screenshot on my initial testing: