Final answer:
Endocytosis is a form of active transport where a cell's plasma membrane folds inward to engulf materials and requires energy to do so. There are three main types: phagocytosis, pinocytosis, and receptor-mediated endocytosis.
Step-by-step explanation:
Endocytosis is a biological process by which cells bring materials into themselves. This form of active transport requires energy, as it involves the plasma membrane of the cell folding inward to form a pocket around the target particle. This pocket eventually pinches off to form a vesicle within the cytoplasm of the cell. There are three main types of endocytosis: phagocytosis, where large particles are engulfed; pinocytosis, which involves the intake of fluid and small particles; and receptor-mediated endocytosis, which is highly selective, allowing specific ligands to bind to cell surface receptors before being internalized. All these forms of endocytosis highlight the cell's ability to actively manage and control the intake of external substances.