What is osmosis and how does it differ from what I learned at GCSE about diffusion

At GCSE osmosis was simplified to just diffusion of water so it is the spreading out of water molecules from where there is a high concentration to a low concentration- but in fact osmosis depends both on the concentration of molecules (ie how much is dissolved in the water) AND what the pressure of the water is (eg when a plant cell is more turgid the pressure is greater)

Solute potential refers to how much solute (eg salt or other molecules) is dissolved in the water and pressure potential refers to what pressure the water is under

Water potential is the net (overall) total of these two factors

Pure water (with no solutes dissolved in it) at atmospheric pressure at sea level (eg just in a glass) has water potential = 0 and it is useful to consider water potential in terms of being more or less negative (as it is generally less than zero)

Adding solute causes water potential to become MORE negative, and increasing the pressure causes water potential to become LESS negative

Osmosis is defined as "the net movement of water molecules from an area of less negative water potential to an area of more negative water potential"- so if the sum of the solute and pressure potential is lower (more negative) in one area than another, there will be a net movement of water molecules into the more negative region (referred to as 'flowing down the water potential gradient')