Stem cells are special cells which have two unique properties, called self-renewal and differentiation.
Self-renewal describes the process by which a stem cell produces an identical copy of itself.
Differentiation describes the ability of stem cells to generate the specialised cell types found in the body, for example blood cells or neurons.
This is achieved through a process called asymmetric division (shown in the figure below) and occurs when stem cells divide. This process allows all the different specialised cell types in the body to be formed, whilst retaining a pool of stem cells.