What is it?

Pinocytosis refers to engulfing macromolecules. A process of taking in fluid together with its contents into the cell by forming narrow channels through its membrane that pinch off into vesicles, and fuse with lysosomes that hydrolyze or break down contents.

Why is it needed by the cell?

It involves the transport of solutes or fluids throughout the cell. It is also the process of cells to ingest the surrounding fluid, but not very specific in the substances or dissolved particles that it absorbs. This process is also called "cell drinking"

What types of molecules move through the membrane this way?

It moves large part particales or whole cells.
external image 300px-Pinocytosis.svg.pngexternal image amoeba17.jpg