A basic knowledge of how bands are formed in crystalline solids is necessary to understand the physics of semiconductors as many of the important members within the class, such as Si, Ge, GaAs and GaN, are crystalline in nature. A crystalline solid can be viewed as a periodic arrangement of atoms/ions, which we call a lattice. While each atom/ion can be perceived as a quantum well. Electrons occupy the eigen states of the well, which are called the orbitals. Interatomic distance is typically of a few angstroms in a lattice. In such a closely packed condition, each atom/ion is influenced by its neighbors, which leads to the modification of the potential profile of individual atoms/ions. The atomic orbitals of neighboring atoms/ions start to overlap with each other, which affects the valence electrons the most. Core electrons, on the other hand, are least influenced as the atomic orbitals are more localized for those states. The valence electrons are no longer confined to individual atoms/ions. They become the part of the whole lattice. The most pertinent question in condensed matter physics is how to obtain the energy eigen states and wavefunctions of the electrons dancing in a periodic array of quantum wells. In order to build the basic formalism, it is necessary to start with certain important terminologies.