Today I have a short script which I cobbled together from Google searches to do screen captures / screen casts with Ubuntu (including audio in so that you can narrate what is going on):

#!/bin/bash
Xaxis=$(xrandr -q | grep '*' | uniq | awk '{print $1}' |  cut -d 'x' -f1)
Yaxis=$(xrandr -q | grep '*' | uniq | awk '{print $1}' |  cut -d 'x' -f2)
avconv -f alsa -i pulse -f x11grab -s $(($Xaxis))x$(($Yaxis)) -i $DISPLAY.0 -r 15 -c:v libx264 -crf 0 -c:a libvo_aacenc -b:a 256k -threads 8 ~/Videos/output.mp4

I found this to work much better than gtk-recordmydesktop, which had lags, especially when “effects” were being drawn on the screen (like bulletpoints sliding in for a presentation or switching between desktops).