* Simple Diffusion: Movement of a substance across a membrane from an area of high concentration to an area of low concentration. This doesn't require energy. Examples: oxygen diffusing into the cell, carbon dioxide diffusing out of the cell.

* Facilitated Diffusion: Movement of a substance across a membrane with the help of a membrane protein. This also doesn't require energy, but it does require the presence of a specific protein. Examples: glucose entering a red blood cell with the help of a glucose transporter protein.

* Osmosis: Movement of water across a semi-permeable membrane from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration). This is driven by the difference in water potential and does not require energy.

Let me know if you'd like more details on any of these mechanisms!