Quantum Chromodynamics (QCD) is the theory of strong interactions and one of the components of the Standard Model. For the benefits of the reader we remind briefly some of its basic features. QCD is the gauge field theory that describes the interactions of colored fermionic fields (quarks) and non-Abelian colored gauge fields (gluons). The (local) gauge transformations form the group (color group) with the quarks and gluons belonging to the fundamental (triplet) and adjoint (octet) representations of the color group . The gluons mediate the interaction between the quarks, and in addition, due to the non-Abelian character of QCD, the gluons also interact with each other. The strength of interaction between colored objects is scale dependent. The renormalized QCD coupling constant is small at high energy (asymptotic freedom); in this domain the fundamental fields of QCD, quarks and gluons, can be probed experimentally in a direct way, and high-precision theoretical predictions using perturbation theory can be confirmed. On the other hand, the attraction between two colored objects grows with increasing distance, that leads to the confinement of quarks and gluons, so that only colorless particles ( singlets) manifest themselves as asymptotic states. At low energy the color degrees of freedom are not directly observable. In this case the hadron dynamics is equivalent to an effective field theory of colorless hadrons.