Want to show someone how to do something on your website, use software, or learn how to use a site like Twitter? Then you want to use a screencasting tool to capture a video of your computer screen as you go through the steps. Jing and Camtasia are the most well-known but Wikipedia has compiled a comparison list of screencasting products that you may use to see what might work best for you.
After you have created a video, you can burn it onto a disc or upload it to a video or screencast hosting site like Youtube, blip.tv, or Screencast.com. If you host the video online you may also embed it into your website or provide a link to it on your Facebook page.