Active galactic nuclei throughout the spectrum : M 87 PKS 2052 47 and the MOJAVE sample Active galactic nuclei (AGN) are one of the many mysteries in the Universe. AGN hosts a super massive black hole (SMBH) in its center surrounded by a rotating accretion disk and a powerful jet is ejected along the pole directions of the accretion disk. The powerful jet is definitely the highlight of AGN. Those extragalactic jets appears to have superluminal motions up to 60 c and they are very luminous and variable across the electromagnetic spectrum. The jets can be highly polarized which indicates the existence of strong magnetic fields; they can have a highly bended morphology due to reasons we do not completely understand. Astronomers have been studying AGN jets for the past decades by performing observations from the radio to the gamma ray band applying physical jet models to the observed properties and trying to put the pieces into the whole picture. We now understand many aspects of the AGN phenomena but a significant part is still missing. The aim of this thesis is to study the extragalactic jets by using different approaches. We investigate: (i) a bright feature in the jet of the radio galaxy M 87 as observed using high resolution VLBI technique; (ii) a multiband flare of the blazar PKS 2052 47 by conducting a multifrequency campaign; (iii) the broadband spectral energy distribution (SED) of a radio selected statistically complete sample of the most conspicuous AGN in the Northern sky: the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) sample ...