if you want to specialize, you submit an application to the application service. Programs that participate in the match rank the applicants they want from most desirable to least and interview applicants. The applicants then rank the program that he/she would like to join (again from most to least). When the list's "match" for both program and applicant the applicant completes his/her residency at the institution that they were matched to.
someone correct me if that was inaccurate.