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We have reviewed a number of recent papers dealing with the most controversial aspects of AGNs. They suggest interesting conclusions:

- All Seyfert galaxies can be essentially described by a single parameter, the X-ray column density which steadily increases from Seyfert 1s to Seyfert 1.8s to Seyfert 1.9s to Seyfert 2s.

- AGNs belong to two main classes: the high-ionization AGNs (Seyfert 1s and 2s) and the low-ionization AGNs (Liners) which probably differ by the accretion rate onto the central BH, Liners having low accretion rates and consequently being powered by an unefficient advection dominated accretion flow.

- HPQs are high-ionization radio loud AGNs with a relativistic jet pointing in the direction of the observer, while BLLs are low-ionization AGNs. Many blazars have been classified as BLLs on the basis of unsufficient data; most objects with weak broad emission lines are HPQs.

- Radio loud AGNs may host a rapidly rotating BH, and radio quiet AGNs a slowly rotating BH. Rapidly spinning BHs could be the result of the merger of two similar mass BHs.

- Many early-type spirals have a nuclear emission line spectrum intermediate between Liners and starbursts (the so-called weak [O I]-Liners); they are probably old starbursts dominated by supernova remnants.

- Cooling flow clusters often show near their center a filamentary structure which has a Liner-like spectrum most probably ionized by stellar processes rather than by an AGN; on the other hand, the central galaxy in these clusters is often a FR I radio galaxy, i.e. a genuine AGN. The true nature of an emission line nebulosity observed in a FR I galaxy centrally located in a cooling flow cluster is therefore somewhat ambiguous.

- ULIGs are powered by starbursts induced by merging processes; many are weak [O I]-Liners. A significant fraction of all ULIGs contains an AGN which is probably the consequence of the merging; the AGN may, in some cases, be the major source of energy.

- Type 2 QSOs exist; they are in general quite inconspicuous as both the broad emission lines and the nuclear continuum are hidden from view. FR II radio galaxies with a high radio luminosity are type 2 QSOs.

Acknowledgement. We are pleased to thank Suzy Collin for constructive discussions and Areg Mickaelian for careful reading of the manuscript.

Table 6. List of the acronyms used

ADAF Advection Dominated Accretion Flow
AGN Active Galactic Nuclei
BAL Broad Absorption Line
BALQSO Broad Absorption Line QSO
BH Black Hole
BLL BL Lacertae object
BLR broad line region
BLRG Broad Line Radio Galaxy
BLS1 Broad Line Seyfert 1
FIR Far-InfraRed
FR I Fanaroff-Riley type I
FR II Fanaroff-Riley type II
FWHM Full Width at Half Maximum
gE giant Elliptical galaxy
HBL High energy peaked BLL
HPQ Highly Polarized Quasar
HST Hubble Space Telescope
HX Hard X-ray (2-10 keV)
ILR Intermediate Line Region
IRAS InfraRed Astronomical Satellite
LBL Low energy peaked BLL
LERG Low-Excitation Radio Galaxy
Liner Low Ionization Nuclear Emission line Region
LX X-ray Luminosity
MB Blue absolute magnitude
MR Red absolute magnitude
MIR Mid-InfraRed
NELG Narrow Emission Line X-ray Galaxy (=NLXG)
NLR Narrow Line Region
NLRG Narrow Line Radio Galaxy
NLS1 Narrow Line Seyfert 1
NLXG Narrow emission Line X-ray Galaxy
PAH Polycyclic Aromatic Hydrocarbon
QSO Quasi Stellar Object or quasar
R Flux ratio of the core to the extended radio components
RS Schwarzschild radius: 2GM / c2
RBL Radio selected BLL
ROSAT Röntgen Observatory SATellite
SMBBH SuperMassive Binary Black Hole
SNR SuperNova Remnant
S1h Seyfert 1 galaxy with a hidden BLR
S1i Seyfert 1 galaxy with an absorbed BLR visible in infrared
ULIG UltraLuminous Infrared Galaxy
VBLR Very Broad Line Region
XBL X-ray selected BLL

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