If you don't know "bokeh" as a vocabulary word, you certainly know their effect: the attractive blurry, bubbley thing that happens to lights in photographs (and movies) with a shallow depth-of-field. In out-of-focus areas, each point of light takes on the shape of the aperture, most often a round disc.
You can actually take advantage of this distortion, to produce these circles on purpose...which can look pretty great.
Tuts Plus offers five quick tips to producing top-notch blurry photos. Check them out!