Osmosis : a diffusion of a solvent (most of the time water molecules) through a membrane that is semipermeable in a area that is low solute concentration to an area of high solute concentration
Diffusion : A net passive motion of particles (atoms, ions, or molecules) from a section which they are higher in concentration compared to areas in lower concentration. It goes on until the concentration is all the same.
Active Transport : The motion of ions or molecules along a cell membrane in the direction opposite of the diffusion or from an area of lower concentration to one of higher concentration. Active transport needs the assistance of a certain protein known as a carrier protein which uses energy supplied by ATP.