A screencast is a video capture of the actions on a user's computer screen, usually with accompanying audio commentary. It is a quick and easy way to explain anything that can be displayed on a computer screen. There are a number of online tools that are free and need no software installation.
You can try it using...
→ screenr.com, or
→ screencast-o-matic.com
1-minute tutorials that double as examples of screencasts...
→ how to user screenr
→ how to use screencast-o-matic
Good further information...
→ Nice article on using screencasting in education is here.
→ Slightly dated (01/2010) comparison of free screencasting tools is here.
