The band gap of a semiconductor is the minimum energy required to move an electron from its bound state to a free state where it can participate in conduction. The band structure of a semiconductor gives the energy of the electrons on the y-axis and is called a "band diagram". The lower energy level of a semiconductor is called the "valence band" (EV) and the energy level at which an electron can be considered free is called the "conduction band" (EC). The band gap (EG) is the distance between the conduction band and valence band.
Once the electron is in the conduction band, it is free to move about the semiconductor and participate in conduction. However, the movement of an electron to the conduction band will also allow an additional conduction process to take place. The movement of an electron to the conduction band leaves behind an empty space for an electron. An electron from a neighboring atom can move into this empty space. When this electron moves, it leaves behind another space. The continual movement of the space for an electron, called a "hole", can be illustrated as the movement of a positively charged particle through the crystal structure. Consequently, the movement of an electron to the conduction band gives results in not only an electron in the conduction but also a hole in the valence band. Both the electron and hole can can participate in conduction and are called "carriers".
The concept of a moving "hole" is analogous to that of a bubble in a liquid. Although it is actually the liquid that moves, it is easier to describe the motion of the bubble going in the opposite direction.