GLUT is Mark Kilgard's "OpenGL Utility Toolkit" library, written to support the sample programs in the second edition OpenGL 'RedBook'. Since then, GLUT has been used in a wide variety of practical applications because it is simple, widely available and highly portable.
GLUT allows the user to create and manage windows containing OpenGL contexts on a wide range of platforms and also read the mouse, keyboard and joystick functions.
The original GLUT library seems to have been abandoned with the most recent version (3.7) dating back to August 1998. Its license does not allow anyone to distribute modified library code. This would be OK, if not for the fact that GLUT is getting old and really needs improvement. Also, GLUT's license is incompatible with some software distributions (e.g., XFree86).
Since Mark Kilgard gets really bothered by idiot GPL zealots who aren't satisfied with "free as beer" software, the open source community has developed a replacement, freeglut. freeglut is a drop-in replacement for GLUT, striving to be a bug-free upgrade.