  This lecture note presents a self-contained introduction to the theory of a
heavy quark-antiquark ($Q\bar{Q}$) system in terms of perturbative QCD. The
lecture is intended for non-experts, such as graduate course students. The
heavy $Q\bar{Q}$ system serves as an ideal laboratory for testing various
aspects of QCD: We can examine the nature of renormalons in perturbative
series; an effective field theory Potential-NRQCD is constructed, whose
derivation from full QCD can be traced stepwise; we see absorption of
renormalons by non-perturbative matrix elements in OPE clearly; a systematic
short-distance expansion of UV contributions can be performed, which predicts a
"Coulomb+linear" potential in perturbative QCD; we can test these theoretical
formulations by comparison to lattice computations, where we observe a
significant overlap with perturbative regime; finally we can test our
microscopic understanding by comparing to experimental data for the bottomonium
states. These subjects are covered in a concise and elementary manner. Overall,
we provide a microscopic description of the main dynamics of a heavy $Q\bar{Q}$
system, as an example for which theoretical framework, practical computations
and qualitative understanding have been most advanced.
