The two types of passive transport are diffusion and osmosis. Diffusion is the movement of molecules across a membrane from an area of high concentration to an area of low concentration, occurring with or without the help of membrane proteins. Osmosis is the diffusion of water molecules.
Movement of water and other molecules across a membrane by passive transport does not require an expenditure of energy. Moving molecules across a membrane from an area of low concentration to an area of high concentration requires the use of energy, and is called active transport. Cell membranes use active and passive transport to maintain cell stability.