What is the difference between diffusion and osmosis?

Osmosis is the movement of solvent particles (usually referring to water) from an area of low concentration (i.e. lots of water particles) to and area of high concentration (i.e. less water particles and more other substances).

Diffusion is the movement of particles from an area of high concentration to low concetration. An example is if a gas canister is opened at one end of the room; because of the differences in concentration the particles of gas will diffuse towards the other end of the room.