The study of a system or phenomenon can be looked upon into matter as a whole or minutely into the behavior of its atoms and molecules. The observation taking matter as a whole or in bulk is said to be macroscopic approach of study. It indicates topics of Mechanics, Thermodynamics, a part of Electromagnetism and Optics. This approach being primitive is also known as Classical Approach.
In the other way around, the study based on microscopic domain i.e. observation of individual atoms and molecules which includes studies like structure of atoms and nuclei, kinetic theory of gases etc. is termed as microscopic or Statistical Approach of study.
Consider a certain amount of gases stored in a closed cylinder. Measurement of its temperature and pressure can be done by fixing a thermometer and pressure gauge through some provisions along walls of the cylinder. Such an approach is macroscopic in nature. It does not require observation of individual atoms or molecules of the matter. Meanwhile, calculating momentum of each molecule of the gas requires determination of velocity with which each of them strikes the walls of cylinder. With the help of momentum data, amount of force exerted by atoms and molecules of the gas can be found out. Summing up force of each atom gives total force acting on walls of the cylinder. Dividing the resulting force with area of walls denotes pressure acting on it. This method of determining pressure took the help of microscopic approach.
It is astonishing to know that the pressure obtained from calculations using microscopic approach matches with outcome of readings of the installed pressure gauge which is macroscopic in nature. This signifies, the output of all entities in microscopic approach can be added up to match with results of the system obtained in macroscopic approach.