Active Galactic Nuclei (AGN) are remarkable astronomical sources emitting over the whole electromagnetic spectrum,
with different bands providing unique windows on distinct
sub-structures and their related physics. AGN come in a large number of types only partially related to intrinsic differences. I highlight here the most important AGN classes, namely jetted and non-jetted, radiatively efficient and inefficient, and face-on and edge-on, the source types
selected by different bands together with the most important selection effects and biases, and the underlying emission processes, emphasising the $\gamma$-ray band. I then conclude with a look at some open issues in AGN research and at the main new astronomical facilities, which will provide us with new data to tackle them.