We begin with a brief overview of magnetic behavior in different types of solids. We first define the terms used to describe the various types of magnetic behavior. A system is called paramagnetic if it has no inherent magnetization, but when subjected to an external field it develops magnetization which is aligned with the field; this corresponds to situations where the microscopic magnetic moments (like the spins) tend to be oriented in the same direction as the external magnetic field. A system is called diamagnetic if it has no inherent magnetization, but when subjected to an external field it develops magnetization that is opposite to the field; this corresponds to situations where the induced microscopic magnetic moments tend to shield the external magnetic field. Finally, a system may exhibit magnetic order even in the absence of an external field. If the microscopic magnetic moments tend to be oriented in the same direction, the system is described as ferromagnetic. A variation on this theme is the situation in which microscopic magnetic moments tend to have parallel orientation but they are not necessarily equal at neighboring sites, which is described as ferrimagnetic behavior. If magnetic moments at neighboring sites tend to point in opposite directions, the system is described as antiferromagnetic. In the latter case there is inherent magnetic order due to the orientation of the microscopic magnetic moments, but the net macroscopic magnetization is zero.