In case you're unfamiliar with the term, a "screencast" is a recording of your desktop. It's an ideal way to demonstrate something, like how to use a complicated feature in Microsoft Office (or how to do file attachments so you don't have to explain it to your mom a 78th time, ahem).
All you do is start your recording, perform the activity one step at a time (narrating along the way if you wish), then stop the recording. The end result is a video you can e-mail to others, post to YouTube, and so on.
You don't need any pricey software or special equipment to create screencasts--just your browser, a few minutes of your time, and a service like Screencast-o-Matic.
It's free. There's nothing to install. And it recently added support for high-definition (1,280x720) recording. You don't even need an account to use it, though you can create a free one if you want to keep tabs on your recordings.
Here's a sample Screencast-o-Matic screencast that shows a brief demonstration of Office 2007:
After you finish recording a screencast, you can save it to your PC in MP4 format or upload it to your YouTube account.
I particularly like the way the service highlights mouse clicks and enlarges the cursor so viewers have an easier time tracking what you're doing.
I should note that if you want to add narration to your screencast, you'll need a microphone. If you have a webcam, you're all set. Otherwise, any old mic will do.