If the medium surrounding the cell has more concentration than the cell, which means the surrounding solution is really dilute, the cell gains water by osmosis. Such a solution is called a HYPO-TONIC solution. As a result, the water from the solution moves into the cell, the cell swells and bursts open (CYTOLYSIS).
If the medium surrounding the cell has lower concentration than the cell, that means it is a very concentrated solution, the cell loses water by osmosis. Such a solution is known as a HYPER-TONIC solution. As a result, water from cell moves to the surrounding solution resulting in the cell to shrink (PLASMOLYSIS).
If the medium surrounding the cell has exactly the same water concentration as the cell, there is no net movement of water across the cell membrane. Such a solution is known as an ISO-TONIC solution.As a result, the water moves to both sides and there is no change in size of the cell.
Hope this helped!
:D