How are the billions of cells in our bodies made? In plants and animals, there are cells capable of producing every type of cell the organism will need from birth to death. Once an organism begins to mature, other cells are required to produce the range of specialized cells needed for a functioning organ or tissue. Such cells are known as stem cells. ‘Stem cells’ explains the two major types of stem cells — embryonic and adult. Embryonic stem cells have different biological properties to the adult stem cells that are found close to and after birth, hatching, or germination. Stem cell properties and cancer stem cells are also described.