A vesicle is a small, membrane-bound sac found within cells. It plays a crucial role in transporting materials, such as proteins and lipids, to different parts of the cell or to the cell membrane for secretion. Vesicles can form naturally during processes like endocytosis, where the cell engulfs external substances, or during exocytosis, where materials are expelled from the cell. Vesicles are essential for various cellular functions, including metabolism and communication. They can vary in size and type, with some specifically designed for storage, like lysosomes, which contain enzymes for breaking down waste, while others, like secretory vesicles, are involved in releasing hormones or neurotransmitters.