GLUT is a library of utilities for OpenGL programs, which primarily perform system-level I/O with the host operating system. Functions performed include window definition, window control, and keyboard and mouse input. GLUT even has some (ugly) support for creating popup windows.
GLUT was written by Mark J. Kilgard, author of 'OpenGL Programming for the X Window System' and 'The Cg Tutorial: The Definitive Guide to Programmable Real-Time Graphics'.
Also, GLUT provides routines for drawing a number of geometric primitives (both in solid and wireframe mode), including cubes, spheres and the Utah teapot.
GLUT is in the first place created to write rather portable code between operating systems (GLUT is available for many operating systems) and in the second place to make learning OpenGL more easy. Getting started with OpenGL programming while using GLUT takes only a few lines of code and you don't need any knowledge of operating system specific windowing API's.
You can recognize a GLUT function by looking at it, they all start with the glut suffix. An example function is glutPostRedisplay() which asks to rerender the current screen.
For more information, see the [http://www.opengl.org/documentation/specs/glut/spec3/spec3.html GLUT documentation].