Diffusion uses the concentration gradient that has been set up, this is a naturally occurring phenomena, and using a diffusion gradient some small particles can cross over the cell membrane. Some bigger or polar molecules require facilitated diffusion to move these molecules across the membrane.
Facilitated diffusion still uses the gradient for passive transport. This means that ATP is not used to transport molecules.
The reason that glucose will not move into the cell via passive transport is because there is a higher concentration of glucose inside the cell, meaning it needs active transport to move glucose (against the concentration gradient) into the cell.
