  In the past decade, exotic hadrons with charm and bottom flavors have been
extensively studied both in experiments and in theories. In this review, we
provide topical discussions by selecting $X,Y,Z$ particles, to which Belle has
made important contributions. These are $X(3872)$, $Y(4260)$, $Z_c(4430)^+$,
$Z_c(3900)^+$, $Z_{b}(10610)^+$ and $Z_{b}(10650)^+$. Based on the current
experimental observations, we discuss those states with emphasis on hadronic
molecule whose dynamics is governed by chiral symmetry and heavy-quark symmetry
of QCD. We also mention briefly various interpretations and some theoretical
predictions for the yet undiscovered exotic hadrons.
