The initial implementation of GLUT was released under a license that prohibited modification. freeglut is a reimplementation of the GLUT API under the MIT license. There have been more freeglut developments and it has API extensions, but in my experience there are some quirks of older versions of GLUT that are not replicated to freeglut. However, if you are just starting to use excess, you can also use freeglut.
But I would not recommend using GLUT or freeglut to manage windows in the first place. In the end, you will be much better off using something like wxWindows or QT.