Osmosis is defined as the movement of solvent particles (usually water) from a region of high concentration to a region of low concentration through a partially permeable membrane. It is often regarded as a form of diffusion in which only water molecules move. So, water moves from an area where it is in high concentration (abundant), to an area where it is in low concentration (less abundant) until both areas have the same concentration of water.
Consider the diagram below where large solute particles are unable to pass through the membrane, it only allows water to pass through. Osmosis occurs until there is an equal concentration on either side (until equilibrium is met).
The Figure above shows Osmosis.
The arrows represent the direction in which water molecules will move across the semi-permeable membrane.
Water has a natural tendency to move from a place where it is in higher concentration to a region where it is of a lower concentration. This tendency is measured as the water potential. Osmosis can be loosely defined as the movement of water molecules from a region of higher water potential to a region of lower water potential. Pure water has the highest water potential. The presence of solute particles (dissolved particles) will reduce the water potential. Because of this, the more solute particles that are present, the lower the water potential of the solution.