Typically, hire no permanent staff of their own other than office and recruiters. Job seeker is on the agency's payroll for duration, need or contract. Paid by the company, not by job seeker. Compensation is contingent upon filling a position. Job seeker pays fee to recruiter to provide employment services.
Different recruiters know about different positions.
They do not usually know about the same ones. By sending your resume out to more than one recruiting agency, you will be placed in many different confidential databases and become aware of many different positions. If you send your resume to only a few, it may be that none you send to will be working with positions which are best suited for you. Remember to throw your net widely. If you change jobs, it is also wise to send follow-up letters to your recruiters to let them know of your new job change.