ReCordMyDeskTop

recordmydesktop is a program that allows you to take a video from your Linux desktop. In practice, you often want to record a window only. To do this, start recordmydesktop like this:

recordmydesktop --windowid $(xwininfo | grep "Window id" | awk '{print $4;}')

A cross will appear. Aim it at the window you want to record and click. Then the recording will start.

Reduce file size

To reduce the video file's size start vlc and convert the video using the menu Media -> Convert/Save.