What are vesicles?

Vesicles are small, spherical sacs found in cells. They are separated from the cytoplasm by at least one bilipid layer. Vesicles may be created by budding off from larger membrane-bound organelles such as the Golgi apparatus or endoplasmic reticulum or by endocytosis of parts of the cell membrane. 

Vesicles function as intracellular transport vehicles, moving various substances around the cell and to the cell membrane. They are also essential for the secretion of molecules and for maintaining the integrity of cellular compartments.