Clients post a freelance project on a bid site and independent contractors with expertise in the project's field then bid on the project. Candidates for the position usually respond by naming a price they would accept for the work as well as by posting their qualifications. Clients then choose a candidate based on their own criteria for price and qualifications (i.e. not necessarily the lowest bidder).
Payment for the work is usually made through the bid site. Bid sites collect a portion of the fee paid for the service.
Bid sites are commonly used for IT, writing, translation and other fields that commonly hire freelancers on a project basis.

