Neutral solutes move across the plasma membrane by the process of simple diffusion along the concentration gradient i.e. from higher concentration to lower concentration. Movement of water by diffusion is called osmosis.
Polar molecules cannot move across the membrane by diffusion. They require a carrier protein of the membrane to facilitate their transport. Few ions or molecules are transported across the membrane against their concentration gradient i.e. from lower to higher concentration which is energy (ATP) consuming and is called active transport.
