A demo/show reel is a movie you create for the single purpose of showing off your work for other people to see. It will contain segments of animations you have done (if you have done any) and/or different shots/poses of models you have created. You should not linger on any one thing for too long because you're only meant to show a very short preview of whatever it is you are showing.

Alias has several reels here: https://www.alias.com/eng/community/m...ry/index.jhtml which you can look at. The "Maya customer Reel" for instance shows off all the products/movies/special effects done with Alias' Maya software.