What is the difference between diffusion, osmosis and active transport?

Diffusion and osmosis both involve the movement of particles from a high concentration to a low concentration, down a concentration gradient. The key difference between the two, is that osmosis concerns the movement of FLUIDS - so the words “water potential” will substitute concentration in that instance. Active Transport is different. It involves the movement of particles going AGAINST the concentration gradient - so from an area of low concentration to high concentration. As the name suggests, this is an active process, so it requires ATP to be able to occur (whereas diffusion and osmosis are both passive processes, that naturally occur without needing any ATP).