When you're looking for a new job, you want to use all the resources available to you. One resource to use can be a recruiter, also known as a headhunter or search firm. Recruiters do not charge candidates for their services.
Companies pay all fees.
Contingency recruiters earn a fee when a client company hires their candidate. Retained recruiters earn a fee, usually paid in installments, whether or not a client company ultimately hires their candidate. Many recruiters offer clients both services, contingency and retained. Most recruiters specialize. Specialties can be industry, discipline, region or any combination of these.