Dropdown terminal is a very useful feature whereby a terminal can pop on the screen just by clicking a button like F12. These are very useful since they provide a quick way to launch the terminal when its used a lot.
To get such a terminal on ubuntu use any of the following tools
Yakuake is a dropdown terminal for KDE desktop. Pressing the F12 button will bring up the terminal.
$ sudo apt-get install yakuake
After installation it can be launched from the shortcut in K > System > Yakuake. To add new Session press Ctrl+Shift+N
To autostart yakuake on system boot add it to the list of statup programs in
System Settings > System Administration > Startup and Shutdown > Autostart
Guake (Quake style terminal) is a dropdown terminal application for the gnome desktop.
$ sudo apt-get install guake
Guake also uses the F12 key by default for launching the terminal, and this can be changed in the Guake Preferences program.
Tilda has a simpler look and is suitable for a desktop environment like xfce.
$ sudo apt-get install tilda