What is osmosis and how does it work?

Osmosis is defined as the movement of water from an area of high water potential to an area of lower water potential, across a partially permeable membrane. This means that water molecules moves from a very 'watery' or dilute solution into a more concentrated area (with a lower water potential.) Water molecules can fit through a partially permeable membrane (like visking tubing) but the other molecules in the solution are too large to fit through the gaps. The molecules move due to what's called the osmotic pressure gradient - the area with more water is under higher osmotic pressure than the area with less water, so molecules move with this gradient.