Kelvian is absolutely correct. The cheapest (read free) recorder out there for the Mac is QuickTime. Not only does it record your screen, it can record your mouse clicks, or a selection of your screen, has keyboard shortcuts to stop recording, can record an audio input (for live voice overs), and has an interface that disappears when you start your recording so it won't be seen in the recording file.
That being said, I have found that in order to capture system sound and audio through a microphone with QuickTime, you'll have to choose Built-In Audio from the twirl-down menu on the screen recorder and hook a male to male 1/8th inch audio cable from your headphone jack to an 1/8th inch splitter or 1/8th inch audio mixer and hook your microphone cable into the 1/8th inch splitter or mixer as well. At this point, you can connect your 1/8th inch splitter or mixer to your microphone jack on your Mac. This happens because the screen recorder in QuickTime 7 only records one audio source at a time. Also, if you're planning on recording video or video games, the QuickTime screen recorder can often get rather choppy (read lower frame rate) so if you're planning on recording those sort of activities, a professional screen recorder would be best.
From experience, I can tell you that the best Mac screen recorder that I've used is Camtasia screen recorder for the Mac. It lets you record mouse movements, mouse clicks, keyboard activity, voice overs, and system sound all at a very high frame rate: http://www.techsmith.com/camtasia.html. It's not free like QuickTime, but it is very affordable at $99.
In addition, iShowU HD has also performed admirably for me in the past. It can do everything that Camtasia does at an even lower price of $60: http://www.shinywhitebox.com/ishowu-hd/. I have found that the frame rate is often a little slower, but only a little slower, than Camtasia so keep that in mind if you're looking into purchasing this product.
Good luck with your screen recording!