|What is copyright?|
Basically, copyright protects the embodiment of an idea in a tangible form. That can mean a published book (or movie, or song, etc.), but it can also be something you've typed on your computer or even scrawled out in less-than-perfect handwriting. The important distinction is that it's tangible; I can print it out or point to it and say, "There it is."
Copyright doesn't protect ideas, however. Let's take this admittedly historically inaccurate situation between two writers:
CHARLOTTE: I believe I shall endeavor to write a book about pride -- and also about prejudice.
JANE: Hold fast, you insipid cow! I have already written such a book. I shall sue the petticoats off of you!
Despite Jane's protestations, Charlotte is actually free to write about pride (and also prejudice) to her heart's content. Why? Because she's working from an idea, not a tangible piece of writing.
What Charlotte cannot do, however, is copy and paste any actual passages from Jane's book or unpublished manuscript. That would be plagiarism, also known as stealing, which would indicate that Charlotte has gone over to the Dark Side and must be dealt a suitably swift and period-specific punishment.
So what does all of this mean to you? First, remember that the moment you start writing a book, it's protected under copyright law. You don't need to stamp the copyright symbol all over it. (In fact, I recommend you don't. It just looks tacky.) Second, if you're really concerned about protecting your work, you can register it with the copyright office. It's relatively cheap and easy, and it gives you much more robust legal protection. Here's the link: http://copyright.gov/
By the way, the term trademark refers to something else entirely. A trademark is something that you use to identify a product or service. It could be a name, a title, a slogan, that sort of thing, e.g. Coca Cola. And a patent applies to an invention or a method of doing something. Neither term is related to copyright, so as long as you're writing a novel and not inventing the next trendy soft drink, you don't really need to worry about those.
The insidious Charlotte, however, might be in for a world of hurt.