A nice feature of GNOME3 is that you can easily create a simple screen recording by pressing Shift-Ctrl-Alt-R (and again to finish the video).
The video file is then located below ~/Videos/ as a .webm file (http://de.wikipedia.org/wiki/WebM). The result is a very compact, high resolution video.
If you want to convert the file to another format, simply use ffpmeg, like
ffmpeg -i VIDEO.webm VIDEO.avi