Charmonium(-like) or bottomonium(-like) states, which are incompatible with
predictions from a static quark anti-quark potential model, are often refered
to as XYZ states. In this lecture, some peculiar properties of states such as
the X(3872), the Y(4260), or the Zc(3900) are explained. Such properties are
utmost proximity to a threshold, overpopulation of states, or possibly binding
in the regime of string breaking. Among decays, the surprising observation of
isospin violation, and using radiative decays for the observation of a D-wave
state is discussed. A second part of the lecture presents recent precision
measurements of masses of newly observed bottomonium states. These masses can
be used for testing particular aspects of the potential, such as test of the
tensor term or test of the flavor independence. At the end, an example is
given, how future experiments may be able to measure widths of a
charmonium(-like) state in the sub-MeV regime.Comment: Lecture given at the Helmholtz International Summer School Physics of
Heavy Quarks and Hadrons, Dubna, Russia, 07/15-28/201
