Purple prose happens when a writer tries to make his writing sound more writerly by using fancy words. The result is that the reader hits a speed bump in the reading road and is yanked up short because the words draw attention to themselves rather than telling the story.
It ain't pretty.
Words should never get in the way of the story we're trying to tell. This little poem has been attributed to either anonymous or Madelaine L'Engle. (Either she wrote it or quoted it from an anonymous source.) It doesn't really matter who said it first; it's the best medicine for the purple prose bug:
The written word
Should be clean as a bone,
Clear as light,
Firm as stone.
Two words are not
So good as one.
The problem with purple prose is that it is not any of these things. When a writer thinks that he's going to juice up the fancy end of his writing a bit so the can impress a publisher or agent, he's probably got the heinous sin of purple prose in mind. Don't try to sound like a writer. Just tell the story in as clear a way as you can, using your own words, simple may they be.
There's a line I like from the movie, The Rocketeer, that can be applied: "Acting is acting like you're not acting. So act!" That's also true about writing. Writing isn't supposed to sound like you're writing. If the reader feels written to, something is wrong, and it's quite possible your purple prose showing.
The right words are important. As Samuel Clemens said, it's either the lightning or the lightning bug. Take your pick. Purple prose is the lightning bug that needs to be squashed.