Critters is an on-line workshop/critique group for serious Science Fiction/Fantasy/Horror writers.
What that means is... A bunch of writers get together, read each others' stories and tell the authors how they felt reading their stories.
The ultimate goal of Critters is to help writers improve, not only by having their work dissected by other members, but also by learning to dissect their own work (by, of course, dissecting others). The value of the latter is often overlooked by beginning writers.
It works something like this: Suppose you submit a short story or chapters of a novel (whole novels are handled differently; see below). You email your manuscript (in the proper format), it gets put into a queue of stories; in about a month, when it bubbles to the top of the queue, it (and a batch of others) are emailed around to the members (or they can get them off the web page). Critiques are due within a week. Most pieces get 15-20 critiques.
"Critique" means an in-depth description of how the critiquer felt about and reacted to the work, with the intent of helping the author improve it. One would likely comment on characterization, plot, setting, logic of the underlying idea, effectiveness of the opening and ending, etc., as well as smaller "nits" like factual inaccuracies, logical mistakes, unclear passages, and perhaps any grammar or spelling errors that escaped proofreading. Comments may be of praise or concern. Generally praise is reserved for areas the reader thinks the author has done exceptionally well. Typically more concerns are raised than praise -- since it's the "concerns" the author is most likely to want to address, especially those shared by multiple readers. Concerns should always be phrased tactfully and as the author's personal opinion.
Critters is large as critique groups go (several thousand members, from ultra-beginner to multi-novel pro), and has handled thousands of manuscripts to date, with enough critique material to fill hundreds of books. Many critique groups come and go, usually failing because of poor organization and member apathy / lack of participation. Critters has successfully solved the first problem by lots of automation :-) and the second by requiring members to submit critiques. Members are asked for roughly one critique a week, with some provision for vacations and such. (You can get ahead, catch up, etc. See the rules for full details.) This has worked phenomenally well.
Critters also has a special program for getting entire novels critiqued quickly, something that is notoriously difficult in any workshop, face-to-face or on-line.
For more information or to join, see the other links off the main page. Your writing will thank you.