Sure. The built-in iSight camera of all Intel iMacs supports both video and audio. The following link shows how to use Quicktime to record video and audio with your iSight:
If you wish to append that video with examples on the computer, SnapZ Pro is a great tool for screen capture:
Note, SnapZ Pro is probably the easiest to record an existing webinar. Note, when recording any webinar, you should be licensed to record the said video, or allowed to by the professor/school offering it for the use of your own instruction.