I've done this professionally for the last 18 months or so and basically how it works is the agency will get in a project, perhaps it's short or long. My contracts range from 3 weeks to 6 months. The agency doesn't want to hire someone on just for this project because maybe they only get a project like this once every 2 years. You get a desk, sometimes a computer, credentials and are basically an employee. You rock up 9-5 like everyone else. At the end of the contract you'll either get extended if they have more work or you'll be onto the next thing. You should be getting paid a bit more for this than if you were an employee because you're often foregoing benefits like sick/annual leave and general job security.