Traditionally, agents are the ones who represent your work to publishers and producers, but lately there has been an upswing in the number of writers' managers. What do they do, and should you have one?
Agents are the ones who negotiate deals for you--in the scriptwriting world, only agents are allowed to do that (not managers). The ideal agent would also concern himself or herself with developing your career, but quite a few of them don't do that. They try to get you as much work as possible and don't worry about the bigger picture. At least in theory, that is where a manager comes in. He or she guides your career, introduces you to the right people, and may have a more personal relationship to you.
It sounds like a great idea, and in some cases it is but of course it comes at a price, usually 15% of your earnings. Add that to the 15% that your agent is taking, and figure maybe another 5% for your accountant and/or your lawyer. Then let the government take their share and you won't end up with very much. And here's the kicker: usually your contract with both the agent and the manager will state that you must pay commissions even if they had nothing to do with bringing you the job.
Naturally, if you can get a manager who is so well-connected and powerful that he or she can help you take your career to a totally new level, it will be worth it. Just be sure that you have a contract that pays on a commission basis only and that you have an out clause for no later than 60 days from signing in case it doesn't work out.