Waylander's post is spot on.
But here's one more point, put simply:
If you want your work placed at a medium-sized or a large house, where it will get promotion (and contrary to popular belief, big publishing houses do promote their products), where it will get read by more people than just your family members, where it will get reviewed and discussed, then you need an agent. An agentless author can get to the point where she places the book at a legitimate house and makes money and gains a readership -- but it is a long, long, long, long shot.
My agent not only proofread my manuscript, she took it to editors that I would otherwise never have been able to contact, she negotiated my contract, she explained foreign rights, the agency hired a foreign rights agent who sold the book all over the world, she brought in a film agent from CAA who got the book read by people like Jerry Bruckheimer. She has now become an advocate for my career, reading manuscripts I send to her for critique and discussing market trends -- stuff like that.
It is possible to make it in publishing without an agent, but highly unlikely.
Oh, and you need to take a trip to the Bewares boards. There are scammers on every streetcorner in this business.
Good luck!