What happens to particles in diffusion?

Diffusion is the movement of particles from a high concentration of said particle to a lower concentration, thus balancing out the difference. The difference between two concentrations is called the concentration gradient (So a high concentration would be a higher difference). If there is a higher concentration gradient, the rate of diffusion will also increase.

Diffusion is the net movement of a substance, this is generally from a area of high concentration to low concentration, the particles will collide or 'bounce' off one another and by random movement, will eventually spread out till they are even.