  Charmonium, the spectroscopy of c\bar{c} mesons, has recently enjoyed a
renaissance with the discovery of several missing states and a number of
unexpected charmonium-like resonances. The discovery of these new states has
been made possible by the extremely large data samples made available by the
B-factories at the Stanford Linear Accelerator Center and at KEK in Japan, and
at the CESR e^+e^- collider at Cornell. Conventional c\bar{c} states are well
described by quark potential models. However, many of these newly discovered
charmonium-like mesons do not seem to fit into the conventional c\bar{c}
spectrum. There is growing evidence that at least some of these new states are
exotic, i.e. new forms of hadronic matter such as mesonic-molecules,
tetraquarks, and/or hybrid mesons. In this review we describe expectations for
the properties of conventional charmonium states and the predictions for
molecules, tetraquarks and hybrids and the various processes that can be used
to produce them. We examine the evidence for the new candidate exotic mesons,
possible explanations, and experimental measurements that might shed further
light on the nature these states.
