Certification programs exist to illustrate that students have a standard skill set that allows them to safely skipper a sailing vessel. The two most prominent in the US are ASA, the American Sailing Association, and US SAILING, both of which are nationally recognized sailing certification and accreditation programs. Similar organizations also exist abroad. These organizations, which include sailing schools, charter companies, sailing instructors, and sailors, set competency standards for both sailing knowledge and on-the-water skills for sailors and instructors from basic to offshore sailing levels.
Certification simply means a person has demonstrated a standard of proficiency by passing both written and on-the-water skills tests (much like a driver's license), and provides a means for charter companies to assess a sailor's ability.
US SAILING has seven experience levels of certification that follow the "building block" technique of compiling learned skills with experience. Once earned, these certification credentials, in conjunction with continued sailing experience, are recognized by charter companies and sailing schools worldwide as the status of your accomplishments in the world of sailing. US SAILING offers courses in Basic Keelboat, Basic Cruising, Bareboat Cruising, Coastal Navigation, Coastal Passagemaking, Offshore Passagemaking, and Celestial Navigation.
The American Sailing Association (ASA) has a similar curriculum, with the identical seven "building block" levels of certification, plus additional certification in Small Boat Sailing, Trailerable Multihull Sailing and Cruising Catamaran sailing. These certifications are also recognized by charter companies worldwide, and will get you onto a boat and sailing away without the lengthy testing and check-out procedures otherwise required. The emphasis is always on safety, and is perhaps a little more geared to the cruising sailor.