Just to add a bit to
sneff's writeup.
In Reggio Emilia in Modena, Italy there is an annual tasting. Balsamico that passes the tasting committe is marked with a red, silver, or gold tag before being sealed with wax. The gold tag balsamico is of the best quality. Even balsamico that comes from Modena that is without these markings is not properly considered balsamic vinegar. (Still damn tasty though.)