A screencast is a digital recording of computer screen output, also known as a video screen capture, often containing audio narration. The term screencast compares with the related term screenshot; whereas screenshot is a picture of a computer screen, a screencast is essentially a movie of the changes over time that a user sees on a computer screen, enhanced with audio narration.
Screencasts can help demonstrate and teach the use of software features. Creating a screencast helps software developers show off their work. Screencasts are a useful tool for ordinary software users as well, to help report bugs (the movie takes the place of potentially unclear written explanations) or to show others how a given task is accomplished in a specific software environment. Screencasts are excellent tools for learning how to use computers, and several podcasts have started to teach computer users how to use software through screencasts.
(from the Wikipedia term Screencast)