http://www.stemcellresearchfacts.org/what-is-a-stem-cell/

A stem cell is essentially a “blank” cell, capable of becoming another more differentiated cell type in the body, such as a skin cell, a muscle cell, or a nerve cell.

Microscopic in size, stem cells are big news in medical and science circles because they can be used to replace or even heal damaged tissues and cells in the body. They can serve as a built-in repair system for the human body, replenishing other cells as long as a person is still alive.