Window Prose is a term I have taken from Brandon Sanderson, which he uses to describe the type of prose he strives to write. The idea is that the writing is so simple, clear, and perfectly-suited to the needs of the scene that the audience doesn't even notice that they're reading. They internalize everything so well, without having to stop to think, that it's almost like they're looking through a window at the action.
Now, there's nothing wrong with being descriptive. We've all come across passages that were simply beautiful, that struck some sort of chord within us and perfectly conveyed a vivid image, emotion, or idea to us. Some situations call for more flowery and elaborate prose than others. The more you write, the easier it will be to tell what sort of prose a scene requires.
But no matter how good a writer you become, you should always err on the side of simplicity and clarity. Your goal should not be to impress your readers with your amazing wordplay, it should be to convey images, emotions, and ideas as precisely as possible. That is what this blog aims to help you achieve.
Now, some examples of purple prose.
From Inheritance by Christopher Paolini:
The branch Roran had added to the fire burst asunder with a muted pop as the coals underneath heated the gnarled length of wood to the point where a small cache of water or sap that had somehow evaded the rays of the sun for untold decades exploded into steam. (pg. 27)
As ImpishIdea points out, this is a very purple way of saying, "the fire crackled."
From Twilight by Stephanie Meyer:
Edward in the sunlight was shocking. I couldn’t get used to it, though I’d been staring at him all afternoon. His skin, white despite the faint flush from yesterday’s hunting trip, literally sparkled, like thousands of tiny diamonds were embedded into the surface. He lay perfectly still in the grass, his shirt open over his sculpted, incandescent chest, his scintillating arms bare. His glistening, pale lavender lids were shut, though of course he didn’t sleep. A perfect statue, carved in some unknown stone, smooth like marble, glittering like crystal. (pg. 260)
In other words, Edward's skin sparkled.
Finally, some deliberate purple prose from Tor.com...
From The Shadow War of the Night Dragons, Book One: The Dead City, by John Scalzi:
Night had come to the city of Skalandarharia, the sort of night with such a quality of black to it that it was as if black coal had been wrapped in blackest velvet, bathed in the purple-black ink of the demon squid Drindel and flung down a black well that descended toward the deepest, blackest crevasses of Drindelthengen, the netherworld ruled by Drindel, in which the sinful were punished, the black of which was so legendarily black that when the dreaded Drindelthengenflagen, the ravenous blind black badger trolls of Drindelthengen, would feast upon the uselessly dilated eyes of damned, the abandoned would cry out in joy as the Drindelthengenflagenmorden, the feared Black Spoons of the Drindelthengenflagen, pressed against their optic nerves, giving them one last sensation of light before the most absolute blackness fell upon them, made yet even blacker by the injury sustained from a falling lump of ink-bathed, velvet-wrapped coal.
With the night came a storm, the likes of which the eldest among the Skalandarharians would proclaim they had seen only once before, although none of them could agree which on which one time that was; some said it was like the fabled Scouring of Skalandarharia, in which the needle-sharp ice-rain flayed the skin from the unjust of the city, provided they were outside at the time, while sparing the just who had stayed indoors; others said it was very similar to the unforgettable Pounding of Skalandarharia, in which hailstones the size of melons destroyed the city’s melon harvest; still others compared it to the oft-commented-upon Moistening of Skalandarharia, in which the persistent humidity made everyone unbearably sticky for several weeks; at which point they were informed that this storm was really nothing like that at all, to which they replied perhaps not, but you had to admit that was a pretty damn miserable time.
Which is to say: It was a dark and stormy night.