What is osmosis and how does it differ from diffusion?

Diffusion and osmosis are very similar processes. Osmosis is a particular case of dissusion, therefore it is easier to explain the overarching theory of diffusion first. Diffusion is a gradual movement of particles from a place of high concentration to low concentration, which in simple terms is particles spreading out because they are free to move about randomly. This can occur in both liquids and gases which brings us to the case of osmosis. 

Osmosis can be viewed as a special incidence of diffusion as it involves a net movement of water molecules across a partially permeable membrane. The important thing to remember is that water moves from a high water concentration orwater potential to a low water concentration/potential. This differs from diffusion as areas with a high water concentration are places of low concentration. The partially permeable membrane mentioned in this definition is a membrane with small holes in it (like a cell membrane), that allows small molecules like water to pass through it.