The point of osmosis is to dilute a solution (increase the water content) so that water balance is restored (same sort of principle as diffusion).
In a solution, the water is known as the solvent while the salt/sugar is the solute.
Water will move through a partially permeable membrane from an area with a high concentration of water to a low concentration of water.
Osmosis in Animals
In an animal, a cell that uses up water in its chemical reactyions will have a more concentrated (less dilute) cytoplasm.
This means that water will move into the cell from outside of the cell as the outside would have a higher water concentration (be more dilute).
This works two ways, if a cell has more dilute cytoplasm inside it, then water will move out of the cell by osmosis.
In both of these cases, water balance is restored by osmosis.
Osmosis in Animals
However, osmosis can sometimes cause problems in cells e.g.
If the solution outside the cell is more dilute than inside the cell, water will move into the cell by osmosis. This can sometimes cause the cell to swell up and burst.
If the solution outside the cell is more concentrated than the inside of the cell, water will move out of the cell by osmosis. This can cause the cytoplasm inside the cell to become too concentrated and the cell may shrivel up and be unable to survive.
Osmosis in Plants
Plants rely on osmosis to support their structure.
Water moves into cells via osmosis. This causes the vacoule in each cell to swell and press the cytoplasm against the cell wall. This makes it so that no more water can move into the cell. The cell will become turgid (firm) which allows the plant to maintain its structure and stay upright.
However, if the solution outside the cell is less dilute, water will move out the cell via osmosis leaving the cell not turgid and unable to support the plant tissues.