There have been a few misconceptions about agents--what they are, what they do, whether they cost or not, how it all works, etc--so I thought it would be good to briefly clarify a few points in case anyone is still unsure.
The number one factor you should be mindful of when searching for an agent is this:
You should not pay agents to represent you / look at your work.
If an agent asks you to pay them up front to read your manuscript or represent you,
run away. Quickly.
The agent gets paid when you sell your book to a publisher. An agent will take a cut out of your royalties that the publisher pays you. Usually this is around 15% - 20% (it can vary, depending where in the world you are and who you sign with).
Again, be extremely wary of agents that ask for payment up front. They could be frauds. If you're not sure, there are a number of excellent websites that list known fraud agencies. Writer Beware is probably the best.
So what are agents and what do they do? Jane Friedman says it clearly and concisely on her website:
In today's market, probably 80 percent of books that the New York publishing houses acquire are sold to them by agents. Agents are experts in the publishing industry. They have inside contacts with specific editors and know better than writers what editor or publisher would be most likely to buy a particular work.
Perhaps most important, agents negotiate the best deal for you, ensure you are paid accurately and fairly, and run interference when necessary between you and the publisher.
It's also advisable to ask an agent who else they represent. Or, you can research this online. Agents unwilling to mention any of their authors by name or any recent sales could be dodgy.
Be wary of agents who refer you to an editing service you have to pay for. As sff.net says:
There is, however, a common scam where the agent recommends an editorial service. There's a good chance the service is paying the agent a kickback to make that recommendation.
Also be watchful for "vanity presses" who expect you to pay them to publish you.
It should be noted that you do not necessarily need an agent. It depends on what publishing route you prefer to take, as well as the type of work you're trying to sell. Not everybody wants an agent or a traditional publisher, and there are other options available, such as self-publishing and e-publishing.