Endocytosis is a cellular process in which a cell takes in substances from its external environment. This occurs when the cell membrane engulfs material, forming a pocket that eventually pinches off to create a vesicle inside the cell. This mechanism allows cells to intake nutrients, hormones, and other important molecules necessary for their function. There are different types of endocytosis, including phagocytosis, where large particles are engulfed, and pinocytosis, which involves the uptake of liquids. Additionally, receptor-mediated endocytosis allows cells to selectively absorb specific molecules by binding them to receptors on the cell surface.