What is osmosis and why does it occur?

Osmosis is the movement of water molecules from an area of high water concentration to an area of low water concentration through a partially/selectively permeable membrane (PPM). It occurs because of concentration gradients; the water moves from the high water concentration area through the PPM to 'balance out' the amount of water on either side of the membrane - that is, to ensure an equal amount of water molecules on each side of the membrane.