It consists of a number of functions that use the base OpenGL library to provide higher-level drawing routines from the more primitive routines that OpenGL provides. It is usually distributed with the base OpenGL package.
Among these features are mapping between screen- and world-coordinates, generation of texture mipmaps, drawing of quadric surfaces, NURBS, tesselation of polygonal primitives, interpretation of OpenGL error codes, an extended range of transformation routines for setting up viewing volumes and simple positioning of the camera. It also provides additional primitives for use in OpenGL applications, including spheres, cylinders and disks.
GLU functions can be easily recognized by looking at them because they all have glu as a suffix. An example function is gluOrtho2D() which defines a two dimensional orthographic projection matrix.
Specifications for GLU are available at the OpenGL specification page