Exotic Discoveries in Familiar Places: Theory of the Onia and Exotics
  This brief overview presents an introduction to the heavy-quark exotic hadron candidates, $X$, $Y$, $Z$, $P_c$, believed to be tetraquark and pentaquark states. After an abridged account of their discovery, we outline the various theoretical pictures employed to describe their structure, with special focus on the new dynamical diquark picture. 
Richard F. Lebed
hep-ph nucl-th