"GNOME 3 has an inbuilt software that allows you to record your desktop. This makes it very easy to create a screencast in GNOME 3. Here is how you do it ...
You can start/stop recording your GNOME 3 Desktop by pressing the Ctrl+Shift+Alt+R key combination.
The first time when you press this key combination, GNOME 3 will start recording your Desktop and any movements that happen on it. A red circle is displayed in the bottom right corner of the screen when the recording is in progress. Pressing the same key combination a second time will stop the recording.
After the recording is finished, a file named 'shell-%d%u-%c.webm' is saved in the home directory. In the filename, %d is the date, %u is a string that makes the filename unique, and %c is a counter that is incremented each time a recording is made within a single gnome-shell session. The recording is saved in the WebM format.
WebM is an open media file format designed for the web. WebM files consist of video streams compressed with the VP8 video codec and audio streams compressed with the Vorbis audio codec. The WebM file structure is based on the Matroska media container."