It's the same process as getting a job by any other means. There's no money out of your pocket.
The major difference is you will interview with people at the company, but all other communication, negotiation, etc is with the recruiter.
They take down either a finders fee or get a scrape on your salary for a certain period of time. You won't see any of that. You get what you are offered. They make 20 to 30% of your salary and pushing for higher pay comes out of that. They don't go back to the company and say you want more, they just eat the reduced margin if they feel like it's worth taking the hit on you to get further business with the company (placing more people gets them more cred with the client).
Be careful when signing anything with them though. The like to toss in little clauses saying things like you are not ever allowed to get a job with any other recruiter in the same industry ever and other dumb things. It's all negotiable so read the contracts.