A retained recruiter is paid in advance (retained) to find individuals to fill a firm’s open position. The retained recruiter is paid even if he/she does not complete the assignment. Usually retained recruiters work with more senior level assignments.
A contingency recruiter is paid only if he/she fills the firm’s open position. It can happen that more than one contingency firm works on a specific assignment at the same time. Contingency firms most likely work on less senior level positions.