Do is an application to find things on your system and quickly perform actions, similar to Quicksilver for OSX and GNOME Launch Box. It works on a variety of different desktop environments, including GNOME and KDE. Basically, all you need to do is to press Super + Space on your keyboard, and the Do dialog will open. Super is the Windows key on a PC, or the Command key on a Mac. Type in the name of an application, and Do will launch it for you.