Radio astronomy has changed. For years it studied relatively rare sources,which emit mostly non-thermal radiation across the entire electromagneticspectrum, i.e. radio quasars and radio galaxies. Now it is reaching such faintflux densities that it detects mainly star-forming galaxies and the more commonradio-quiet active galactic nuclei. These sources make up the bulk of theextragalactic sky, which has been studied for decades in the infrared, optical,and X-ray bands. I follow the transformation of radio astronomy by reviewingthe main components of the radio sky at the bright and faint ends, the issue oftheir proper classification, their number counts, luminosity functions, andevolution. The overall "big picture" astrophysical implications of theseresults, and their relevance for a number of hot topics in extragalacticastronomy, are also discussed. The future prospects of the faint radio sky arevery bright, as we will soon be flooded with survey data. This review should beuseful to all extragalactic astronomers, irrespective of their favouriteelectromagnetic band(s), and even stellar astronomers might find it somewhatgratifying.
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