Strangely,  Ubuntu 14.04 LTS (and previous versions, down to 10.04), loses it’s alt-tab feature for switching between windows. A feature which you definetely want back, as you use it constantly.

Fortunately it’s very easy to get the feature back:
Launch a terminal and install the CompizConfig Settings Manager, by typing:
“sudo apt-get install compizconfig-settings-manager && sudo apt-get install compiz-plugins”

Start the CompizConfig Settings Manager, by typing: “ccsm”.

In the left pane, navigate to “Window Management”. Remove the checkbox (if checked) for the Application Switcher, and check the box for “Static Application Switcher”.

You should now be able to <alt+tab> your way through your windows again.