GTK+ is an
object-oriented widget kit.
Considering they managed to do this in
C, that's quite an acheivement.
(Hint: Macros...thousands of 'em)
If you've ever written any code in AWT or other GUI toolkits, it should be quite easy for you to pick up. There's an excellent tutorial at the GTK+ website (www.gtk.org)
GTK+ is used to build the GUI for the GNOME desktop, and there are also a lot of pure GTK+ apps out there.
The best thing about GTK+ is it's themability - GTK themes are much more versatile than KDE ones (although KDE 2.0 has better theming than KDE1.X, the beta version I tried recently became unstable running with flashy themes).