The writing is over: now what?

For the next few posts, I'll talk about what happens when you've finished writing your manuscript. To make things easy I'll break it into a few parts.

1. How do you know when you're really finished under your contract?
2. Is it really you who gets to decide what goes into the manuscript?
3. On what basis can the publisher reject your manuscript?
4. What do you do if the publisher rejects your manuscript?
5. Once they've accepted your manuscript, do they have to publish the book?

Note: even though I'm talking about books and manuscripts, the rule applies for pretty much any creative content: films, video games, works of art, etc.