Stem cells are one of several varieties of cell that have the ability to renew themselves through division, and differentiate themselves into specialized cell types.
There is a gradient that corresponds to how well a particular stem cell can differentiate.
The two most common types, adult and embryonic, are multipotent and pluripotent respectively. Multipotent means that a stem cell can differentiate into similar types of cells, for instance blood cells only. Pluripotent means that they can turn into any of the 3 germ layer types.
Even though embryonic stem cells can differentiate into more cell types than can adult stem cells, like with in vitro fertilization, they require the destruction of the embryo, i.e. abortion. After harvesting embryonic stem cells, they can be cultured for an extended period of time and induced to proliferate without the need to destroy more embryos, though stem cell lines tend to drift genetically and become unusable after years of continuous culturing. New human embryonic stem cell lines are generated from the discarded embryos of in vitro fertilization clinics rather than the fertilization of human eggs for the express purpose of producing stem cell lines. There is some research that shows that embryonic stem cells might be obtainable without destruction of the embryo, though this research is not definite yet.
Largely driven by federal bans on using NIH funds to produce new human embryonic cell lines, much attention has been shifted away from embryonic stem cells to induced pluripotent stem cells (iPS cells). This type of cell is generated from adult (differentiated) cells though genetic manipulation and possess many of the properties of embryonic stem cells. iPS cells are a major leap forward in stem cell research and provide the distinct advantage of generating pluripotent stem cells from any adult individual. Thus, replacement cells and tissues generated from iPS cells can be immunologically compatible with the recipient if the iPS cells were initially germinated from cells donated by the recipient; this significantly reduces the risk of tissue rejection. This advancement in the stem cell field would not have likely come to pass this soon if not for the push to ban human embryonic stem cell research in the US.
There are five categories of stem cells that correspond to their origin in the body: