Did you know you can record screencasts using QuickTime? Me neither, but you can.
1. First, select File > New Screen Recording from the menu:
2. You should see this little thing pop up:
3. Press the record button and you'll be presented with instructions:
4. At this point you can record the entire screen or a just a selection. You can record from any display that's hooked up to your Mac. To end the recording, click on this icon in the menu bar:
5. After recording, save the video by going to File > Export and selecting one of the options:
Of course, there aren't a lot of editing options in QuickTime. It's limited to pretty much rotating and trimming the video, but it gets the job done.
For more advanced screen recording software, I use ScreenFlow. It's easy to use and has a ton of features that will make your screencasts look amazing.