Guake is a top-down terminal for Gnome (in the style of Yakuake for KDE, Tilda or the terminal used in Quake).
To install Guake open your Ubuntu Software Center (Applications -> Ubuntu Software Center) and search for guake. Select the search result and click on the Install button, when installed close your Ubuntu Software Center. To start Guake go to Applications -> Accessories -> Guake Terminal. When Guake is running press F12 to open or close your Terminal window.