Osmosis is the net movement of water from an area of low to high concentration of solutes through a semipermeable membrane. If none of the compartments contains solutes, then the water moves in either direction between the compartments. However, if we add a solute to one of the compartments, this will affect the probability of water molecules leaving that compartment and moving into the other compartment. The ability of water to move into or out of a cell is called tonicity. The tonicity of a solution is related to its osmolarity, which is the total concentration of all the solutes in the solution. A solution with low osmolarity has few solute particles per liter of solution, whereas a solution with high osmolarity has many solute particles per liter of solution. When two solutions with different osmolarities are separated by a membrane permeable to water but not permeable to solutes, water diffuses from the side with lower osmolarity to the side with higher osmolarity. So, solutions can be:

Hypotonic: The extracellular fluid has a lower osmolarity than the fluid inside the cell, it is hypotonic with respect to the cell, and the net flow of water will be into the cell.
Hypertonic: The extracellular fluid has a higher osmolarity than the cytoplasm of the cell, it is hypertonic with respect to the cell and water will flow out of the cell.
Isotonic: The extracellular fluid and the cell have the same osmolarity so there is no net movement of water.

If a cell is placed in a hypertonic solution, water will leave the cell and the cell will shrink due to the difference in pressure and may even die from dehydration.

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