YouTube Video Download Tool
If you find a youtube.com video you like, you can bookmark it using your browser; you can link to it from a webpage; and if the owner has permitted, youtube provides an 'embed code' so that you can paste a code snippet into a webpage and embed the video in a context other than youtube. There are advantages to having people view a video in the youtube interface, such as the ability to make comments, but of course there are also disadvantages, such as the ability to make comments! (sic)
If you want to actually download a video from youtube, you will need a third party program, of which there are several, here is one:

http://www.techcrunch.com/get-youtube-movie/"To download a video to your computer, enter the YouTube URL for the video in the box above (example). It will be downloaded in flv format - use VLC or another compatible player to view it."
VLC is VideoLAN - VLC media player:
http://www.videolan.org/