They are all writers' critique sites, peer-review systems variously sponsored by publishers, arts councils or other sources of funding. The idea is that you upload a portion of your work-in-progress and receive feedback, in return for which you offer feedback on other writers' work. The big shiny carrot is often the "chance of publication" by one means or another; although many may appear to be misleading and offering false hopes to writers, it's often more the case that writers view these sites through rose-tinted glasses and expect (or feel entitled to) a great deal more than is actually on offer. Having said that, some of the sites below may offer assisted publication of one sort or another, or links to CreateSpace and other print-on-demand suppliers.
I've been a member of all 4 above at different times with varying levels of participation. Bear in mind that these are just my opinions based on my own participation - sites change and evolve and you may have altogether different experiences.
YouWriteOn started up in 2006 with Arts Council funding. It's a quid pro quo system - you review a randomly-assigned book and earn a "credit" which you can spend to get a review of your book. You read your assigned extract, write a review of a minimum length and then answer author-set questions (designed to "prove" that you have actually read the extract, although it's really very easy to fool the system by copying the text into word and searching for key terms). You also have to award stars for different elements of the book (characterisation, plot, dialogue etc). Pros: It's hard to game the system or mutually-inflate your mate's book, since you can't choose what you read. Cons: You can't choose what you read. Also, in the time I was there, there were dubious levels of consistency in the stars awarded. Carrot: Review of top-scoring books by leading publishers/agents.
Authonomy started in late 2007 in beta with 100 users, then opened to all in 2008. Owned and run by HarperCollins UK. Here you can read, comment, vote and shelve what you like when you like - and others can do the same for you. Books move up the chart on a complex algorithm based on how often your book is on how many bookshelves and the "talent-spotter" rating of the shelf-owner! Pros: Feedback can be very detailed and constructive. Cons: Easy to game system, plus feedback can often be little more than variations of "Loved it. Shelved it. Can you please shelve my book too?" Carrot: Review by HC editor for top books, and several books from site have been picked up by HC and other publishers for traditional publication.
Bookcountry is Penguin's offering. Similar in operation to authonomy, you can read and comment on whatever you wish. As an author you can set topics for feedback and get stars awarded for these topics. Books become buzz books and favourites and top-scoring books are featured on the site. Pros: A good place to maintain a presence to attract readers. And feedback can often be detailed and useful. Cons: No real "charts" to climb, if that's what you're after. Carrot: Various agents and editors allegedly scout the site.
Most of the above sites have forums of a sort. Authonomy is the only forum I've participated in to any extent. It's most definitely toxic at times (but then I'm seeing more and more internet flame-wars these days, particularly where writing is concerned), but I have to say I've made some wonderful friends there, many of whom I'm sure will remain friends for a long time. Many of them are also talented writers, whose work I have enjoyed reading.
And if you're a reader, but not a writer, these sites are still open to you. Feel free to look around and comment on other peoples' books. Pure readers are often like gold dust - they have no ulterior motives and nothing to gain by giving good or bad reviews so are often seen as far more honest than the writers.
You have nothing to lose except time!