More on diffusion..
As Elm Tree points out, diffusion is:
'The movement of particles from an area of high concentration to an area of low concentration.'
Diffusion is a vital process for life. It keeps every cell in your body alive by allowing cells to get nutrients from and lose wastes to surrounding fluid. It makes your muscles work. It makes the air around you breathable. It makes your hands puffy if you stay in the bath for too long etc. Basically, no diffusion, no life.
So, how does diffusion work? Everything you see around you is made of molecules and atoms. These particles have kinetic energy (ie, they bounce around randomly). This means that over time, you get a homogeneous spread of particles around their container. From the outside, it looks like particles are moving from an area of high concentration to an area of low concentration though in fact, particles are moving in all directions randomly all the time.
Diffusion can occur across membranes. A specialised form of diffusion is called osmosis.