Diffusion of water across a differentially permeable membrane is called osmosis.
Osmosis is a type of passive diffusion where the solvent (water) moves across the membrane, rather than the solute.
The Effect of Osmosis on Cells
Osmosis can affect the size and shape of cells, depending on differences in water concentration across the membrane. You can think of this as the cells gaining or losing "pressure" like miniature ballons.
Cells placed in an isotonic solution do not change because the concentration of water on both sides of the membrane is the same.
Cells placed in a hypertonic solution lose water because the concentration of water is higher inside the cell and water rushes out.
An animal cell in a hypertonic solution shrinks.
A plant cell in a hypertonic solution undergoes plasmolysis (shrinking of the cytoplasm) and the plant often wilts.