Can you explain to me the difference between diffusion, osmosis and active transport?

The key thing here is to note that both diffusion and osmosis are passive processes, and active transport is an active process. A passive process is one which does not require any energy in order to occur, and an active process does require energy.  Diffusion is simply the movement of any molecule from an area of higher concentration to an area of lower concentration, down a concentration gradient. These can be any molecules in a gas or a liquid. Think of it as though the molecules are simply moving to somewhere where there is more space for them to spread out.  Osmosis is a special type of diffusion and can only be used to describe the movement of water molecules (as oppose to another type of molecule such as a gas). Osmosis occurs where there is a semi-permeable membrane which only allows water molecules through. On one side of the membrane there are more solute molecules than there are on the other side (one side has a high concentration of solute). Water molecules move across the semi-permeable membrane from the less concentrated solution to the more concentrated solution, evening out the concentration gradient. They do this until equilibrium is reached and the concentration gradients on either side of the semi-permeable membrane are the same.  Active transport requires energy. This is because in active transport, a substance is moved from an area of low concentration to an area of high concentration, against the concentration gradient. This is the opposite direction to diffusion and osmosis. Think of it like a fish swimming upstream in a river, which requires lots of energy!