Let's start out with their own description:
This site is dedicated to showing demonstrative videos produced by our community. For many subjects seeing something done is the most effective way to learn, whether it's peering over an expert's shoulder while they explain how to program or watching while someone grinds beans and makes coffee. Please watch, learn and perhaps share your knowledge back.
It is a community site where community members can request howto/tutorial videos and other community members make them and put them up on ShowMeDo. They have areas for Python, Java, Linux, Ruby, Blender, and Screencasting. There is also an "Others" section.
Community members can vote on a video's worth and they have a forum for discussion. They also have a blog and a wiki.