When it comes to energy bands, we are actually referring to the energy around the electrons in a certain object. Electrons are known have the negative charge and that is why there are different types of energy bands that can be used for different purposes. Different solids have different types of energy bands though the bands are usually consistent of electrons- how they fit in is what makes them different. Think of this in terms of weight in a person. A and B may be twins but B may be heavier and have different traits where weight is concerned because of how he is made. This applies to objects too. Usually different types of energy bands are found around the atom. These bands are charged and how a certain object acts and behaves internally is said to be affected by the various types of energy bands.
The different types of energy bands are referred to as “models” in most cases because of the fact that these stereotypical types of energy bands have structures that are applicable to different types of solids and different theories are applied to these as well. The KKR model is one of the many types of energy bands that are currently known of and as per this model the sphere in the object are not overlapping over one another and the potential found inside an electron is said to be symmetric to the nucleus of the structure.
Then, there are those types of energy bands that are found in Mott insulators which mostly have a band structure that is free of electrons but they predict the same number of electrons in singular cells. Where the number of electrons in these types of energy bands is not even, it is presumed that one of the electrons has been left unpaired and so, since the band is not complete, the object becomes a conductor rather than what it was initially supposed to be- an insulator. Despite this the object becomes an insulator where this specific object is concerned. This is one of those types of energy bands that are known as conductor bands in which the valence electrons are not attracted to the nucleus to a strong extent and are loosely around them. The electrons are free electrons and where the energy bands are empty or not filled, that makes affects their functioning and makes them insulators instead.
There is the forbidden energy types of energy bands which are actually not energy bands at all. Neither positive nor negative energy can be found here. Think of this as an energy vacuum where no energy is found at all. When the energy gap is too much then, the valence electrons are even more tightly attracted and packed around the nucleus but in order to fill these types of energy bands one can always push the electron to the valence electrons so that they can create a charge in the bands which would result in the filling of the energy bands.