The movement of substances across plasma membrane is classified into active transport and passive transport. Passive transport is further classified into diffusion and osmosis.
Active transport
Active transport is the movement of substances against concentration gradient, from a region of low concentration to a region of high concentration. Active transport involves carrier proteins, requires energy expenditure and the presence of membrane is not necessary. For example, movement of solutes and ions.
Diffusion
Diffusion is the movement of substances follows concentration gradient, from a region of high concentration to a region of low concentration. Diffusion does not require energy expenditure and the presence of membrane is not necessary. For example, movement of liquid or gas.
Osmosis
Osmosis is the movement of water molecules follows concentration gradient, from a region of high concentration to a region of low concentration. Osmosis does not require energy expenditure and the presence of semi-permeable membrane is necessary. For example, movement of water molecules.