Regulation (EU) No 1025/2012 defines a standard as a technical specification adopted by a recognised standardisation body for repeated or continuous application, with which compliance is not compulsory, and a harmonised standard as a European standard adopted on the basis of a request made by the Commission for the application of EU harmonisation legislation (Article 2(1)(c)). The European standardisation organisations are CEN, CENELEC and ETSI. Under Article 10(6), where a harmonised standard satisfies the requirements it aims to cover, the Commission publishes its reference in the Official Journal of the European Union.
That publication is what gives the standard its legal effect. The product directives and regulations provide that a product which conforms to a harmonised standard, or parts of it, whose reference has been published is presumed to conform to the essential requirements covered by that standard. The standard itself remains voluntary: a manufacturer may use other technical solutions, but then has to demonstrate conformity by other means and, under some acts, must involve a notified body.
In practice a manufacturer lists the harmonised standards applied in the EU declaration of conformity and in the technical documentation, and keeps track of new editions and of changes to the Official Journal listing, since only the listed references give presumption of conformity. The General Product Safety Regulation applies the same mechanism to non-harmonised consumer products: Article 7 provides that a product conforming to a European standard published under that regulation is presumed to meet the general safety requirement.