What is osmosis?

To understand osmosis, first you need to understand water potential. Pure water has a water potential of 0, and as it becomes less pure (due to things dissolving in the water) the water potential goes down into the minus numbers. Osmosis describes the tendency for water to want to move from a high water potential where is purer to a lower water potential. Water does this because it is trying to create an equilibrium - where the water is the same purity on both sides.