The three main types of endocytosis are all membrane transport processes that involve the inward movement of molecules into a cell. They are categorized by the mechanism of vesicle formation:

* Phagocytosis: This is the process of engulfing large particles, such as bacteria or cellular debris. It involves the formation of large vesicles called phagosomes. Function: Primarily for defense and removal of cellular debris.

* Pinocytosis: This is the process of engulfing fluids and dissolved solutes. It involves the formation of small vesicles called pinosomes. Function: Primarily for uptake of nutrients and fluids.

* Receptor-mediated endocytosis: This is a more specific process that involves the binding of ligands to receptors on the cell surface, triggering the formation of coated pits and subsequent vesicle formation. Function: Specific uptake of molecules such as hormones, vitamins, and iron.

All three types of endocytosis are active transport processes, meaning they require energy to occur. This is because they involve the movement of molecules against their concentration gradients.