What is endocytosis, and what are its three main types?

Endocytosis is a process by which cells engulf substances from their external environment by invaginating a portion of the cell membrane, forming a vesicle or vacuole. Its three main types are: 1) Phagocytosis ('cell eating'), where large particles or whole cells are engulfed. 2) Pinocytosis ('cell drinking'), where the cell takes in extracellular fluid and dissolved solutes. 3) Receptor-mediated endocytosis, a highly specific process where cells take in specific molecules that bind to receptors on the cell surface.