A pinocytotic vesicle is a structure that aids in transportation as a cell's plasma membrane absorbs molecules from outside. Dictionary.com describes this process as taking place when the exterior membrane enfolds the molecule prior to transport into the cell's cytoplasm.
Nonspecific absorption by the cell can be accomplished in different ways, depending on the substance being absorbed, as explained by Wikipedia. One way of taking in fluids is pinocytosis, or "cell drinking," as it is sometimes called. In this process, the surface of a cell invaginates to form a pocket. Extracellular fluid is then drawn into that pocket as it closes to form a pouch. This pouch then pinches shut at its apex and detaches from the rest of the cell's plasma membrane, whereupon it is conveyed into the cytoplasm of the cell as a pinocytotic vesicle.
Once adrift in the cell, the pinocytotic vesicle synthesizes very small amounts of ATP from lipids that have been freed from the point of detachment, according to Wikipedia. This ATP is useful to the pinocytotic vesicle as a source of energy that fuels the reactions by which the substances within the vesicle are broken down. The process is mostly useful for the absorption of extracellular fluids as it makes no distinction regarding the substances that it takes up.