1. Embryonic Stem Cells: These are derived from the inner cell mass of a blastocyst, which is a very early stage embryo. Embryonic stem cells are pluripotent, meaning they can differentiate into any cell type in the body. This makes them highly valuable for research and potential therapeutic applications. However, their use raises ethical concerns due to the destruction of embryos.

2. Adult Stem Cells: These stem cells are found in various tissues and organs throughout the body. They are multipotent, meaning they can only differentiate into a limited number of cell types specific to their tissue of origin. Examples include bone marrow stem cells that produce blood cells, and neural stem cells found in the brain. Adult stem cells are used in some medical treatments, such as bone marrow transplantation.

3. Induced Pluripotent Stem Cells (iPSCs): These are adult cells that have been reprogrammed to become pluripotent, similar to embryonic stem cells. This is achieved by introducing specific genes into the cells. iPSCs offer a promising alternative to embryonic stem cells, as they avoid the ethical concerns and can be derived from a patient's own cells, reducing the risk of immune rejection.