Novel, novella, short story? The word count difference in writing

Two posts in one day? Have the clocks been destroyed? Has my face melted into an Escher painting?

Word count is something that worries all writers or, if not, it probably should. Is it a novella? A short story? A microwaveable napkin?

OK, so for some definitive answers I suggest you trundle onto this post here by D. Robert Grixti, and it is definitely recommended – by me anyway, and what higher authority is there?