Stem cells are a type of precursor cell that have not been differentiated into specialized cells, or in other words they are a class of undifferentiated cells that have not been specialized to become/have a special function in any organ or tissue. There are two main sources of stem cells.
The first source is the embryo (the fetus): the embryonic stem cells.
The second source is the adult cells/tissue in an adult’s body. The adult stem cells have been mostly found in bone marrow, and then in liver, nerve tissue, skin, and can exist throughout the body.