There are two main types or forms of stem cells—embryonic stem cells and adult stem cells.

Embryonic stem cells (EMCs) are derived from early-stage mammalian embryos that are no more than three to five days old. EMCs are capable of dividing into more stem cells and transforming into multiple types of cells in the body, which is why they're known as pluripotent stem cells. They are mainly used in scientific studies and research, as well as in medicine—for tissue repair and regeneration. The use of EMCs in scientific research is sometimes considered controversial, however, mainly by religious people and institutions, because it destroys human embryos.

Adult stem cells are undifferentiated cells that can be found anywhere in the body and can also be used in medicine for cell and tissue regeneration and repair. Unlike embryonic stem cells, adult stem cells have a more limiting ability to divide into other types of cells, or they can only transform into some cells of the body, not all of them (for example, a stem cell that comes from the blood can only divide into more blood stem cells). Thus, adult stem cells are known as multipotent.

The adult stem cells that can be changed or modified to be more similar to embryonic stem cells in a lab via genetic reprogramming are known as induced pluripotent stem cells.