If you have put your resume up on Monster or CareerBuilder, you may receive phone calls from firms out there that place candidates at companies for a fee.
Job headhunters, also known as recruiters or job search consultants, are hired by client companies to find qualified candidates for positions within the client company. The client company pays their fees, which are commission based.
In some cases a company will have a staff of recruiters and headhunters looking for qualified candidates. If you find a good head hunter, they can be invaluable to your job search. Don't completely depend on them though. You should never solely depend on one source for prospective jobs.