Animals have two main types of stem cell. Embryonic stem cells come from early embryos. They can differentiate into almost any type of cell in the body. Adult stem cells are found in adult tissues such as bone marrow. They can usually only differentiate into a limited range of cell types, often just the types found in their own tissue. Bone marrow stem cells, for example, make blood cells.
Embryonic stem cells could be used to treat diseases, as they can be grown and turned into many different cell types. However, getting them involves using human embryos, which some people object to on ethical grounds. Adult stem cells can be taken from the patient, which avoids this problem, but they are limited in the cells they can become.
Scientists carry out research into the different types of stem cell to find out what they can become and how they might treat conditions such as diabetes and paralysis.