  The past decade witnessed a remarkable proliferation of exotic
charmonium-like resonances discovered at accelerators. In particular, the
recently observed charged states are clearly not interpretable as q-qbar
mesons. Notwithstanding the considerable advances on the experimental side,
conflicting theoretical descriptions do not seem to provide a definitive
picture about the nature of the so-called XYZ particles. We present here a
comprehensive review about this intriguing topic, discussing both those
experimental and theoretical aspects which we consider relevant to make further
progress in the field. At this state of progress, XYZ phenomenology speaks in
favour of the existence of compact four-quark particles (tetraquarks) and we
believe that realizing this instructs us in the quest for a firm theoretical
framework.
