I'm an IT contractor and have worked through various agencies.

The way it works is they're basically the middle man between you, and the person/business you work for.

They deal with everything related to you for the most part, absences, payroll, HR, etc. In IT you might use your own Ltd company, or you may get paid via an umbrella company - not sure what the deal is outside of IT.

Jobs are usually FTC on an hourly rate and to get paid you submit weekly time sheets then you're paid the following week, usually.

One thing to be wary about is typically your take home pay will be lower than you expect, as typically the advertised rate will pay for your wage, your income tax and NICs, as well as employer NIC's and any umbrella and agency fees - this is why many contractors have their own Ltd company, it allows them to get paid what was advertised.