Clathrin-coated vesicles are small, membrane-bound sacs that transport molecules within cells. They form when a protein called clathrin coats the inner surface of a cell membrane, creating a pit that eventually pinches off to form a vesicle. This process is essential for moving proteins, lipids, and other substances between different compartments in the cell. These vesicles play a crucial role in endocytosis, a process where cells take in external materials. Once formed, clathrin-coated vesicles can fuse with other organelles, such as endosomes, to deliver their cargo, helping maintain cellular function and communication.