Diffusion is the movement of particles down their concentration gradient. In other words, diffusion occurs when particles move from an area of higher concentration to an area of lower concentration.
Diffusion is a form of passive transport, which implies that it does not require the use of energy.
This movement is the result of the random kinetic motion of the particles within a substance. Kinetic energy is the energy of motion. All particles are in constant, random motion.
On the other hand, energy is required in the form of ATP (adenosine triphosphate) when particles move from areas of low centration to areas of high concentration. This form of transport is known as active transport. When occurring in a cell, active transport utilizes a protein carrier that is embedded within the cell membrane.