Entropy measures the amount of disorder in a physical system, such as the arrangement of molecules in a gas or atoms in a crystal. Entropy is calculated by finding the change in heat within a system and dividing it by the absolute temperature.
Entropy is measured via units of heat and temperature because on the molecular level, heat and temperature are the result of particles moving and bumping into one another. Temperature is the total kinetic energy in a system, whereas heat is the individual transfers of energy between moving particles.
As the particles in a liquid or gas move about, they tend toward arrangements with the highest probability of occurring. Stable, orderly arrangements like cubes or piles are much less likely than disorderly jumbles because cubes and stacks can only be built one particular way, while jumbles can be created many different ways. When a system moves toward these arrangements of higher probability, its entropy increases.