The article contains descriptions of formal, non-formal and informal learning of adults. The author presents a history of the formulation of the ideas and theories that were fundamental for distinguishing particular types. The main phenomenon in learning of adults is an 'iceberg'. The article outlines a fourth definition of non-formal education and explains how different criteria's influence distinctions between the other types of education. In conclusion the author presents definitions of formal, non-formal and informal learning of adults as seen by international organizations.