The description of a physical system requires the knowledge of some information, for example the prediction of the motion of a point particle in classical mechanics requires the knowledge of its initial position and momentum besides, of course, the system of forces acting upon it. In the case of a great quantity of particles, i.e. of the order of the Avogadro’s number (6.022 × 1023), the information for a detailed description of the motion of every particle are practically not available; therefore distribution functions and statistical methods are introduced in order to describe the behavior of complex systems.