A stem cell is an undifferentiated cell of an organism which is capable of giving rise to many more cells of the same type, and from which certain other cells can arise from differentiation.
Stem cells from human embryos can be cloned and made to differentiate into most different types of human cells
Stem cells from adult bone marrow can form many types of cells including blood cells
Meristem tissue in plants can differentiate into any type of plant cell, throughout the life of the plant.
Treatment with stem cells may be able to help conditions such as diabetes and paralysis