Seyfert 1s are normal spiral or E galaxies with a compact, starlike nucleus and a nuclear emission line spectrum characterized by broad (a few thousands km s-1) permitted lines and narrow (a few hundreds km s-1) high excitation lines. Seyfert 2s have a narrow emission line spectrum like the Seyfert 1s, but they are lacking both the compact nucleus and the broad emission lines. Quasars (or QSOs) are high luminosity Seyfert 1 nuclei (MB < -24; throughout this paper, we use H0 = 50 km s-1 Mpc-1 and q0 = 0.5); their luminosity is so high that the host galaxy is difficult to detect. Seyferts and QSOs contain a compact nuclear continuum source ionizing a broad line region, surrounded by an optically thick torus of dust. Depending on the orientation of this torus with respect to the line of sight, the central object is seen or hidden; when it is hidden, we see only the narrow, extended emission line region; the galaxy is a Seyfert 2. The major breakthrough in understanding the connection between Seyfert 1s and 2s was the discovery by  of a ``hidden'' broad line region (BLR) in the Seyfert 2 NGC1068.
Many spiral galaxies contain a starburst in their central region. Spectra of narrow line Seyferts and starbursts are easily distinguished by their main emission line ratios. But diagnostic diagrams built with these emission line ratios reveal a third type of emission line spectra called Liners (Low Ionization Nuclear Emission line Region). Some of these objects are most probably low luminosity Active Galactic Nuclei (AGNs); others must be related to the cooling flow phenomenon occuring in clusters of galaxies or are produced in the collision and merging of gas rich galaxies.
Figure 1. Schematic diagram of FR II active nuclei showing how the appearance can change from NLRG (Seyfert 2) to BLRG (Seyfert 1) to HPQ with changing viewing angle.
Seyferts and QSOs can be radio loud or radio quiet. Radio loud objects are always hosted by an E galaxy. Most radio galaxies have a double lobe structure; the high radio luminosity sources have edge-brightened lobes; they are called FR II radio sources (for Fanaroff-Riley type II). The low luminosity sources are called FR Is. FR II radio galaxies have the nuclear emission line spectrum of Seyferts; when they have broad emission lines they are called Broad Line Radio Galaxies (BLRGs); when they have the emission line spectrum of a Seyfert 2, they are called Narrow Line Radio Galaxies (NLRGs). All radio quasars have a FR II morphology. FR Is have a weak low excitation emission line spectrum, similar to Liners or no detectable emission at all.
The lobes of radio galaxies (FR Is and FR IIs) are powered by a relativistic jet; when the angle between the jet axis and the line of sight is small, the jet is Doppler boosted by a large factor and the whole spectrum (from radio to -ray) is dominated by a compact, highly polarized, highly variable, superluminal, almost featureless continuum. These objects are called blazars; they are divided into two subclasses: the Highly Polarized Quasars (HPQs) which show broad emission lines, and the BL Lacertae objects (BLLs) with no or weak broad emission lines. The parent population of the HPQs is made of the FR IIs, while the parent population of the BLLs is made of the FR Is. Fig. 1 is a schematic diagram of an FR II galaxy showing how the appearance changes with the viewing angle.
The most popular explanation for the AGN powerhouse involves accretion of gas onto a supermassive, perhaps spinning black hole (BH). Different regimes of accretion have been invoked to constitute the basis of a unified picture of AGNs. The predictions of the theory are that rotationally supported thin disks would form at lower accretion rates (M < MEdd), while supercritical (M MEdd) accretion flows are expected to form thick disks supported by radiation pressure. A very subcritical flow may not be able to cool and, instead of forming a thin disk, it puffs up giving rise to an ion torus supported by gas, rather than radiation, pressure (; ).
This is, very schematically, the generally accepted Unified Scheme of AGNs.
A number of reviews have been published in recent years dealing with various aspects of this topic: a brief history of AGNs (), emission line regions (), continuum radiation (), ultraviolet and optical continuum emission (), emission lines (; ), BALQSOs (), X-ray properties (; ), high-energy radiation (), variability (; ; ), parsec-scale jets (), structure () and polarization () of extended extragalactic radio sources, Liners (Heckman 1987; ; Ho 1998), BLLs (; Kollgaard 1994), blazars (), accretion disks (; ; ; ) and black holes in galactic nuclei (; ), the torus model (Antonucci 1996), the unified scheme (Lawrence 1987; ; ), physical processes in AGNs (), ionized gas in E galaxies (), luminous infrared galaxies (), interacting galaxies (),...
In the following, we review recent work which has brought some light on several as yet unanswered questions: the nature of Liners, the distinction between HPQs and BLLs, the basic differences between high- and low-ionization radio galaxies and between radio loud and radio quiet AGNs, the source of energy powering ULIGs, the existence of type 2 QSOs, ...