Nathan Bransford, one of my favorite agent-bloggers, wrote a good piece on agents. You should seriously read it. "What Do Literary Agents Do?"
So. You want to get that 100,000 word abomination you call a "novel" published. Lucky you, it's not a jungle out there. There are a few things you should know before going out there and trying to get it published.
0) Nobody buys 100,000 word novels as your first novel. Edit it down to under 100,000.
All jokes aside, a rough guide to publishing follows.
1) There are five general "types" of publishing. This is my own breakdown of them, there are sources that would disagree on them, and people who ask "Why the hell do you separate them like that, Andre, you jackass?" But l will continue with what I've got, because I don't care to go back and edit int.
Large Publishing House - These are the big dogs with several imprints to their name and huge market presence. While they are slowly in decline due to a reluctance to adapt to the Internet quickly, they are still a formidable concentration of money. Not to mention, they have huge marketing departments and a host of other cool things. To get into these, however, you need an
agent. More on them later.
Small Publishing House - These are smaller groups, generally trained around a niche market, or with some business practice that is different from normal houses, but if they have sold a good amount of books before, then you can be sure that they don't fall into the last category. They may or may not require an agent to get into, depending on the publishing house. If they're a startup, then trust your instincts. If they sound shady, don't do business with them, that's a good way to get burned. Be cautious!
Self-Publishing - Maybe you think you can beat the system. Maybe you will. There are a lot of stigmas out about those who self-publish, but those who do it well (note: These are few and far between!) can end up with more focus put on them and their self-promotion, and can potentially get offered an agent or a bump up to a bigger publishing house. With self-publishing, sometimes you can mark up your own book to a point, but for the most part, it's harder to snare a profit this way.* Also of note is that getting self-published through an internet company like Lulu can get your book posted on Amazon, but it will not show up in brick and mortar stores. This is because these books cannot be sent back to the publisher.** If you're not printing it yourself, it's wise to go with a service that charges the customer for the printing fees. This is what separates the self-publishing world from the next bit, vanity publishing. Oh. And do try to get something with an ISBN. This is about the only way works of poetry are going to get published. Or large collections of short stories for new authors.
Vanity Publishing - This could hardly be called publishing at all. You send a document to a printer, tell them how many copies, and pay them for the copies. This will often set you back quite a ways. But, hey, you own all copies of the book, and can sell them as you wish. They are not listed anywhere, like print-on-demand services. Of course, this is terribly expensive. Vanity publishing is often used for personal things, family reunion books and the like. It used to be used often for books of poetry, where that now falls to self-publishing/print on demand programs.
Scam Publishing - Okay, this is what happens sometimes. That's okay, it happens to a lot of people. You paid in advance, went with a new group, or didn't listen to that nagging feeling in the back of your head that said "Do not pursue." Happens to the best of us. Still, scam publishing is where you paid for something and received no product. Report them to the authorities in their area. Scamming is pretty damn illegal. If you want your money back, and happen to live near a place that has a court show, go on that. Unless you paid an obscene amount. Then get a lawyer.
Okay, so that's an overview of publishing. Now, what's an agent and why do you want one?
Agents are magical book fairies that know the ins and outs of the labyrinth of publishing and contract law. Everything in the previous sentence is true except for the words between "Agents" and "know." To be technical, they are partners in the business of writing. They help you get your book published, and to make you become a star, and you help them by raising them up to where they're known as the awesome guy who was totally your agent, so they can represent more books.
Agents will not charge you up-front. They take their money as a percentage of sales, generally 10-15% on domestic sales. Take it as a warning sign if they want you to pay them some absurd amount. Agents offer a lot of advice, and help manage other things as well, like foreign rights to books, movie rights negotiations, and other helpful things. They can also offer indispensable advice and help you find ideas when you're running low. Agents can be a great help.
That isn't to say there aren't bad agents out there. If an agent requires you to pay money before sending them a submission, then they're probably not legit. Also, if you see them advertising in a lot of places, they're probably not going to be much good. Ultimately, ask to see a list of books they've represented. If they don't show it to you, run like hell. If they say they represent poetry, then that's another red flag. No one in their right mind tries to sell poetry.
Ultimately, I provide you with a link to
Writer Beware. It's an amazing page, For more information, read it as much as possible. It also gives a lot of good examples.
* If you're getting into writing to score profits enough to live off of, you'll probably be disappointed.
** New technology alert: They have made a device which can make print-on-demand books in the bookstore in ten to fifteen minutes or so. This may affect the future of self-publishing.
Novelist Bunnies: Do you write in order? 
