Recruitment agencies are firms who go and find candidates for the employers. If you get a job through a recruitment agency which is requested by the employer you become an employee of the employer.
Agencies get lists of job vacancies from brokers or to their counterpart manpower agency abroad from employers. They place advertisements on the internet, classified ads, flyers, job boards, and sometimes in other places. They are always on the lookout for new CVs or resume being uploaded to the internet job sites.
Recruitment agencies are getting paid by the employer for every candidate they put forward who successfully gets a job chosen by the employer. Some agencies get paid on a fee basis, and some agencies get paid a fee based on your starting salary.