A resident who wants to become a fellow must complete residency training and become eligible for board certification. A fellowship program, on the other hand, is legitimized either by accreditation or certification. The difference between these approval processes is important to understand.
Accreditation is a voluntary peer-reviewed process (e.g. accreditation by Society of Urologic Oncology, Society of Endourology, etc.), whereas certification is a formal standardized evaluation process designed to ensure that “a certified medical specialist has successfully completed an approved educational program (e.g.certification for Pediatric Urology by the ACGME).” Until recently no subspecialty certification was available in the field of urology. Certification in pediatric urology was launched in 2008. All other currently available urologic fellowships are accredited—not certified—programs. To make things more confusing, some well-respected fellowships are unaccredited altogether. Importantly, a physician who has not been certified or accredited in a particular subspecialty still can practice within that subspecialty. For example, a general urologist is not in any way prohibited from resecting a malignant renal mass nor repairing hypospadias. The details of the application process, as well as which fellowships offer accreditation/certification are constantly evolving. There are also many subtleties with fellowships that aren't evident until you begin asking the right questions. These include what type of call you will take (if any), whether or not you will have attending privileges, and whether there is any opportunity for a salary bonus if you reach a certain billing level (rare, but it does exist). It may be relevant to determine where the money to pay your salary is coming from (particularly if the fellowship has a research component--i.e. will you be expected to apply for a grant?).
Urology Fellowship Guide