Screencasting is the recording of a computer screen (with voice over) as a video that can be shared and made available on the internet. There are lots of free and paid for tools you can use - a few are listed below.  

Screenr Screenr is very simple web based software that enables users to record their computer screen with voice over which can then be saved and shared with others.

Screencast-o-MaticScreen recording software Screencast-o-Matic enables users to record their computer screen and upload the recording to the internet. You can add captions to the video, share it, upload it to YouTube or save the video as an MP4, AVI, or flash video.

CamStudio (Windows) – Free Windows only, open-source, screencasting software. It is easy to use, good if you just want a quick recording from your screen