Gtk+ is a framework for building platform-independent graphical user interfaces. This is a setup program for installing Gtk+ Version 3.6.2 and all of its dependencies on Windows.
The installer must be run as Administrator.
The installer sets up everything required for running Gtk+ and gtkmm based applications. See here for more details.
See here for the developer version of the package (includes headers etc.).
See here about how to create your own Gtk+/gtkmm applications.