ScreenCasts generally consist of a video of either a portion of the desktop or a full screen where the author demonstrates some application or process. ScreenCasts are great for showing people how to do specific tasks on a computer. They are especially useful for new users where one of the first stumbling blocks can be a lack of familiarity with the product look & feel, location of options and general operation.
ScreenCasts may optionally contain an audio track describing what is happening on the screen. The audio element can help to supplement and describe the features being demonstrated on the screen. An audio description can also be used to pad out the sections of the video where nothing interesting is happening. Examples of this dead-time include as a client waits for a response from a server, as source code is compiled, or whilst an application starts up.
Whilst it is possible in most screencasting applications to record the video and audio simultaneously, this often leads to lower quality results. If the author is trying to demonstrate a task, remember the steps they want to demo, and talk coherently then often it is the latter that suffers. Many audio tracks on screencasts have lots of "umms" and "ahhhs" which can detract from the learning experience.
Arguably it is better to record the audio after the video, then subsequently merge them. This allows the author to concentrate on the demo when recording the video, and focus on the audio later.
There is an (almost) complete tutorial available about recording the screencast and merging it with audio recordings later. You can find it here: ScreencastTeam/RecordingScreencasts
Audio recording applications
The audio track on a screencast is optional, many examples to be found online have none. They often have a terminal or notepad open on the screen were the content author "narrates" the video by typing comments rather than speaking.
Video editing / post-processing / format conversion
Methods of recording
ScreenCasts/RecordingCurrentDesktop - Recording whatever you have on your screen right now.
ScreenCasts/RecordingVirtualMachine - Using QEMU / kvm to emulate another system, and recording that.
ScreenCasts/RecordingVNCSession - Recording a local or remote session started by VNC.
These are included here for inspiration purposes.
http://screencastsonline.com/sco/ - Mac tutorials