What is the difference between diffusion and active transport?

Diffusion is the movement of particles from a high to low concentration and does not require energy. An example of diffusion is oxygen moving from the airways (high concentration inhaled) to the lungs (low oxygen in the lungs). Active transport is the movement of particles from an area of low to high concentration. This process requires energy in the form of ATP. For example plants use this when taking up nutrients from the soil. The soil has a low concentration of nutrients, whereas the plant has a high concentration (because it has taken so much up) so it uses energy to take up more nutrients.