Osmosis

Osmosis is the net movement of water molecules across a differentially permeable membrane from a region of a higher water potential to a lower water potential.
Water potential of pure water is 0, so other solution have a negative number of water potential. A solution which the water potential is lower that the cytoplasm of the cell is regarded as hypertonic solution. Solutions which the water potential is higher than the cytoplasm of the cell is regarded as hypotonic solution.
For hypertonic solution, there is a net movement of water molecules across a differentially permeable membrane from the cytoplasm of the cell to the solution by osmosis. The animal cell will shrink and become wrinkled and plant cell will become plasmolysed and flaccid.
For hypotonic solution, there is a net movement of water molecules across a differentially permeable membrane from the solution to the cytoplasm of the cell by osmosis. The animal cell will swell and finally burst and plant cell will become turgid.
If the water potential of the cytoplasm of the cell is equal to the water potential of the solution, there will be no net movement of water molecules across a differentially permeable membrane by osmosis, the solution is called isotonic solution.
Osmosis is important for water into and exit the cells. Especially important for the movement of water from one cell to another in plants. Osmosis is also important for transporting water through living tissues.