  We review theoretical aspects of quantum chromodynamics (QCD) at finite
temperature. The most important physical variable to characterize hot QCD is
the Polyakov loop, which is an approximate order parameter for quark
deconfinement in a hot gluonic medium. Additionally to its role as an order
parameter, the Polyakov loop has rich physical contents in both perturbative
and non-perturbative sectors. This review covers a wide range of subjects
associated with the Polyakov loop from topological defects in hot QCD to model
building with coupling to the Polyakov loop.
