There is a large diversity in the nature of the observed radiation from different AGN. To bring order to the subject, an extensive observational classification scheme has developed. In this section, the scheme is briefly presented. Various properties of observed AGN are described in parallel to the classification scheme. To aid clarity, only the modern form of this scheme shall be addressed. The detailed observational properties of different AGN can be found in Woltjer (1991).
A fundamental division in the classification scheme, which probably relates to a fundamental physical difference, is based on the radio-properties of AGN. A large majority of AGN fall into one of the categories of radio-loud objects or radio-quiet objects (see Della Ceca et al. 1994 for a recent demonstration of the radio-loud/radio-quiet dichotomy). About 10 per cent of AGN are radio-loud with the remaining being radio-quiet.