In this image we have two spheres - a bright sphere peeking from behind a nearby dark sphere. The glare shader creates a flare and glow around the bright sphere in a physically accurate way. As the bright sphere comes further into view, the glaring becomes more pronounced.
A nice feature of the glare shader is that the user can paint his own streak patterns for the flares. Above we see the star lens streak map, glare also comes with a streak map that simulates the streaking of a human eye.
A QuickTime movie (here) shows how the glare changes with motion of the spheres.
Download the scene.