Our current understanding of the strong interaction of hadrons is that they are described by QCD, but we know very little about the physical states of the theory. Until we can predict the properties of the physical states of the theory and confirm these predictions by experiments, we can hardly claim to understand QCD, which has implications beyond hadron physics. Some essential properties of colour dynamics (QCD) and the salient features of quark model for hadrons are also described. The basic approach of quantum mechanics like general consequences of the Schrodinger equation, Virial and Feynman-Hellmann theorems, Quantum Mechanical sum rules, Bound state normalization are discussed. The semiclassical results are hold for potentials which are simple powers of the interquark separation or depend logarithmically upon the separation. We have discussed mass dependence of Schrodinger wave function and power-law potentials for difference potential index. The last chapter is devoted to the leptonic and radiative decay widths of quarkonia.