Afther the discovery of J/ψ and ψ′ mesons many other narrow resonances were found which represent various levels of a system called charmonium. In this review theoretical interpretation of charmonium properties is given. The classification of charmonium levels and their electromagnetic and strong decays are discussed. The basic assumption is that strong interaction between charmed quark and charmed antiquark in charmonium is mediated by coloured gluons as described by Quantum Chromodynamics (QCD). It is argued that charmonium is an ideal object to test the basic ideas of QCD. Both the traditional nonrelativistic approach to charmonium and the dispersion one are presented. The latter approach is free from many additional assumptions inherent to potential calculations. Other applications of QCD to the charmed world are briefly reviewed. Different parts of the review are addressed to different readers. It could, in particular, be of interest to students, specializing in elementary particle physics, experimentalists and theoreticians who are active in this field. The final results are presented in such a form as to follow for a direct comparison with corresponding experimental data.