In the last few years there is a term that seems to have come into general use in Photography - Bokeh. Rumor has it that it is an Englishification (it is too a word) of a Japanese term for blur - 'boke'. The 'h' was added so that English speakers would not pronounce it to rhyme with 'broke' - it sounds more like 'bouquet'. Anyway.
Bokeh is a term that gets misused a lot - so here is the definition from Wikipedia to set it straight: In photography, bokeh is the blur, or the aesthetic quality of the blur, in out-of-focus areas of an image, or "the way the lens renders out-of-focus points of light." Differences in lens aberrations and aperture shape cause some lens designs to blur the image in a way that is pleasing to the eye, while others produce blurring that is unpleasant or distracting— "good" or "bad" bokeh, respectively. ....
Bokeh is often most visible around small background highlights, such as specular reflections and light sources, which is why it is often associated with such areas. However, bokeh is not limited to highlights, as blur occurs in all out-of-focus regions of the image. (Edited by me for content and appearance)
MOST lenses, when shot wide open, will produce shots with 'acceptable' bokeh, but the appearance of the out of focus elements in a photograph is always subjective - and what may be pleasing to one person may not, in fact, be pleasing to others. However, to most eyes, out of focus specular highlights rendered as small circles are usually much more pleasant than those same elements would be if they had apparent edges or corners.
There are many elements of a lens and the way it's constructed that have the potential to affect bokeh, so when buying a lens for your camera, test shoot first, and see what wide open photos look like. Of course, that's easier with Digital than film, but less and less film is being shot nowadays, and fewer film cameras are being purchased, so i guess I'm really directing my comments at those shooting digital. If you're buying a lens from a reputable store, more than likely they'll allow you to shoot the lens anyway. If they don't want to let you shoot the lens - then caveat emptor. Let the buyer beware. Me, personally, I won't buy a lens I can't shoot - and I don't care if I happen to be buying the lens from a boutique camera shop or Best Buy. And by the same token, it's why I don't buy lenses online.
But that's just me. I'm weird that way.