Ligand:A molecule that binds specifically to another molecule, often larger one. In this case, it would be what binds to our receptor protein on our cellular "door".
Vesicle:Small structure within a cell consisting of fluid enclosed by a lipid bilayer. They are used for transporting materials within cells - think mini delivery trucks!
Clathrin: A protein that plays a major role in the formation of coated vesicles, which are used to transport molecules between different areas within the cell.