Osmosis

     Osmosis is the movement of solvent molecules through a selectively permeable membrane into regions of high concentration to regions of low concentration. Diffusion is the spread of particles through random motion from regions of high concentration to regions to low concentration. The difference between them  is that osmsosis is the diffusion of water only, throught passive transport and diffusion is the transport of varies substances by the means of passive or active transport. In diffusion and osmosis any passive transport can happen without energy since they don't need ATP. A concentration gradient is when there's an uneven distribution of a substance across a border. An exmaple: If we picture each individual molecule as a little blue dot and there's a thousand of them. On one side there is 850 little blue dots and the other side is 150, the sides  aren't even. Solute & Solvent have to do with Osmosis and Diffusion because osmosis is the diffusion of solvent through a permeable membrane. Water is not always a solvent  because not all things dissolve in water.