Osmosis is a special case of diffusion.
Osmosis is the movent of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration.
1) A partially permeable membrane is just one with very small holes in it. So small only tiny molecules like water can pass through them, and bigger molecules like sucrose can't.
2) The water molecules actually pass both ways through the membrane during osmosis. This happens because water molecules move about randomly all the time.
3) But because there are more water molecules on one side than on the other, there's a steady net flow of water into the region with fewer water molecules, i.e. the stronger sugar solution.
4) This means that the strong sugar solution gets more dilute. The water acts like it's trying to "even up" the concentration either side of the membrane.
5) Osmosis is a type of diffusion-passive movement of water particles from an area of high water concentration to an area of low water concentration.
Water moves into and out of cells by diffusion
1) Tissue fluid surrounds the cells in the body- it's basically just water with oxygen, glucose and stuff dissolved in it. It's squeezed out of the blood capillaries to supply cells with everything they need.
2) The tissue fluid will usually have a different concentration to the fluid inside the cell. This means that water will either move into the cell from the tissue fluid, or out of the cell, by osmosis.
3) If a cell is short of water, the solution inside it will become quite concentrated. This usually means the solution outside is more dilute, and so water will move into the cell by osmosis.
4) If a cell has lots of water, the solution inside it will become more dilute, and water will be drawn out of the cell and into the fluid outside by osmosis.