The Gnome and KDE interfaces can run on the same computer, even both together on a user desktop, and applications can run within each other’s desktops. But they look different and are built using competing development platforms; KDE is built with the Qt toolkit, while Gnome is built with the GTK+ toolkit. While each has strengths, they are largely redundant, duplicating most of each other’s functions. So they are really competitors. As a developer, you would have to choose which toolkit to develop an application with. As a user, you will probably lean toward one or the other most of the time.