A screencast is a video of your screen with narration. You can then share the video on YouTube or a similar site.
Microsoft's Expression Encoder 4 is a good free screencast tool. It outputs WMV files in 1080p high-definition. These can be easily uploaded to YouTube.
The free version of Encoder 4 has a 10-minute limit per screencast. But that's close to the YouTube length limit anyway. But you can add narration and splice in previously-recorded footage.