An introduction to active galactic nuclei: Classification and unification

In this article I present a summary of AGN classification, followed by a critical review of attempts to understand aspects of the classification in terms of orientation-based unified schemes. Concentrating on radio-loud AGN, I show that the unified schemes based on anisotropy induced both by beaming in relativistic jets and by absorption in dusty torus structures, work well in a broad-brush sense. However, they represent simplifications of a situation that is, in reality, likely to be more complex. In particular, the AGN selected in radio flux limited samples encompass a wide range of intrinsic X-ray/UV/optical/infrared properties for a given radio power, reflecting a number of variable factors in addition to orientation (e.g. large scale environment, accretion rate onto central black hole). There is also a growing recognition that AGN are dynamic, evolving objects that strongly influence their immediate surroundings, including the distribution of circum-nuclear gas and dust.