Metacity is the default window manager for the Gnome desktop environment(KDE uses Kwin). GTK (Gimp Tool Kit)is a toolkit used to create user interface components(GUI). Metacity uses the GTK toolkit to create it's GUI interface(KDE uses QT). A Metacity theme usually (not always) will contain changes for the window decorations and the GTK theme, so all of the GUI will change. GTK themes usually do not contain changes for the window decorations, and will only affect the GUI for colors, menu bars, sliders, etc. A GTK theme engine is the brains behind a themes attributes. If you want to install a GTK theme based on Aurora, you need to have the Aurora engine installed to use it. Most of the popular theme engines are in the repos.
"The only thing necessary for the triumph of evil is for good men to do nothing." Edmund Burke