recordmydesktop is pretty good. You can easily get it started at the command-line by just typing the command. Just put focus back on the terminal and press
Ctrl+C to stop it again, or you can configure a keyboard shortcut on the command line. See
On any Debian-based system, install it like this:
sudo apt-get install recordmydesktop gtk-recordmydesktop
You may also want to record just a specific region of the screen like this:
recordmydesktop -x 100 -y 100 --width 200 --height 50
Or just run
gtk-recordMyDesktop and click+drag on the little preview image that appears in the recordMyDesktop window, or click "Select Window" and click on the window itself.