A snippet is a piece of dialogue or scene I've written which concerns a particular character or perhaps thought about putting into the story. Sometimes it is simply back story; sometimes it's a piece of a scene that helps define character or back story but either doesn't move the story along or I took the story a different route. I don't pay a lot of attention to 'cleaning it up' because it's never meant to appear in the first place. It is always rough draft form. Sometimes I am thinking with my fingertips on the keyboard, formulating as I go. I actually write that way. OK, I do correct the spelling. I have a thing about words being spelled incorrectly. Don't get me started!
An excerpt is a piece from a work in progress. What I post as an excerpt is at least a first revision, not a rough draft. I have cleaned it up and it is part of the story. What I try to do is have enough so it does make sense even if I did lift out of the original context.
My recent snippet from Rain--"Judith" is definitely rough and a little rambling. What it holds is back story and a look into what motivates my male character. The excerpt from Snow is an actual conversation taking place between my female character and her brother. The dialogue shows, not just background for the main story, but part of the plot development.
Now I shall have to think for awhile. Snippet or excerpt? Rain or Snow? I can't decide. Would anyone care to express an preference? Go ahead. It's easy.
First drafts are for learning what your novel or story is about.