Introduction

The 1993 edition of the catalogue of quasi-stellar objects by Hewitt and Burbidge contains 7315 quasars and BL Lacs, which is an order of magnitude more than the number included in their first catalogue, published in 1977. Many more quasars have appeared in the literature since the publication of the 1993 catalogue, and the number continues to increase as observational and data analysis techniques become more efficient. Another catalogue due to Hewitt and Burbidge (1991) contains 935 galaxies that have nuclei with properties similar to those of quasars. The large numbers of quasars and galaxies with active nuclei (AGN) were discovered because of their peculiar colours, morphology or strong emission in the radio, X-ray or infrared bands, or in one of the surveys specially designed to identify objects that have properties different from Galactic stars and ‘normal’ galaxies. A small number of quasars have also been found through their variability or lack of proper motion and these provide samples independent of the biases inherent in the other techniques.

We shall consider in this chapter the various techniques used in the identification process, but first we provide brief definitions of some of the types of object that appear in the book. The definitions do not provide a water-tight division into physically distinct classes of object, and there are many marginal cases that could belong to more than one class. We shall elaborate on the definition and properties of the different kinds of object in other chapters.