What different desktop environments and shells are available?
I'm new to Ubuntu (and linux in general). Some times when looking at programs in the Ubuntu Software Center, I see things about the programs being 'for the GTK+ environment', or 'for GNOME', or a whole lot of other things. All I know is that I've got Ubuntu 12.04 LTS, and what I'm seeing through my monitor appears to be the controversial Unity interface.
What are these other things I see mentioned in the descriptions of programs (such as KDE, GTK, QT, GNOME, GNOME 'shell', etc.)? Are they Linux GUIs that are alternatives to Unity? Are they dependencies that I need to have?
I've spent the last several months getting used to Ubuntu in its present form (though I confess, I have played around with Compiz, which is fun as hell). Will these other things completely turn my world upside down if I use them?