In statistical physics the concept of an ensemble is one of the cornerstones in the definition of thermodynamical quantities. An ensemble is a collection of microphysics systems from which we derive expectations values and thermodynamical properties related to experiment. As an example, the specific heat (which is a measurable quantity in the laboratory) of a system of infinitely many particles, can be derived from the basic interactions between the microscopic constituents. The latter can span from electrons to atoms and molecules or a system of classical spins. All these microscopic constituents interact via a well-defined interaction. We say therefore that statistical physics bridges the gap between the microscopic world and the macroscopic world. Thermodynamical quantities such as the specific heat or net magnetization of a system can all be derived from a microscopic theory.