Endocytosis is an active cellular process that involves the engulfment of extracellular material by the cell membrane. It is distinct from passive transport, which relies on concentration gradients for molecule movement. Endocytosis requires energy from GTPases and can be classified into phagocytosis (large particle uptake), pinocytosis (fluid and small molecule uptake), and receptor-mediated endocytosis (specific ligand recognition). During receptor-mediated endocytosis, clathrin-coated pits form, triggered by the binding of ligands to cell surface receptors, initiating the internalization of specific molecules.