Embryonic stem cell research is the utilization of stem cells from embyros which can then be grown in a laboratory culture and produce specialized cells supposedly to treat diseases or used for research purposes. A common source of such cells is discarded embryos at fertility clinics. The use of embryonic stem cells in medicine has met with some controversy, particularly from the pro-life movement that is concerned about the destruction of embryos that they believe constitute human life.
Adult stem cell research is similar except it does not use destroyed embryos, and instead is based on stem cells that can be taken from adults. These cells are less versatile than those harvested from discarded embryos, but they do not raise the associated ethical dilemas.
The primary reason for wanting to use embryonic stem cells is that they are capable of producing any type tissue in the body. Thus, theoretically, a stem cell could be used to generate a new liver, or that same cell could be used to generate brain tissue. The failure of this argument lies in the fact that no one knows how to control the cell and make it create the desired tissue. Thus it becomes hit or miss, and there is a tendency for stem cells to create "monsters" commonly found in humans and called "teratomas". These mostly consist of hair and teeth. Only when "control" of the cells can be estabalished can there be reliable use of any kind of stem cell. If and when that "control" is established, it would make more sense to use adult stem cells from a persons own body so there would be no problem with foreign body DNA.
Arguments for and Against Embryonic Stem Cell Research
A variety of arguments are used both for and against embryonic stem cell research. Some proponents argue that the embryos do not constitute human lives or that even if they do since the embryos would be destroyed anyways and thus it makes sense to use the embryos.. These arguments have been accepted by some Christian groups and by many Jewish groups, even Orthodox ones..
Opponents to such research have argued that such embryos are human lives with the full rights of humans and that even if they are not human lives the slippery slope is too great. Furthermore, they argue that the sanctity of human life is so great that even the destruction of such embryos for research is not permissible even when they would be destroyed anyways. Opponents have also argued that the potential of embryonic stem cells have been exaggerated and that more research must be done with cord and adult stem cells., , .
An alternative approach would be to use parthenogenesis, the term that's applied to an egg that activates spontaneously on its own. It is common for eggs to activate and often form cysts or benign tumors in the ovary. When the activated eggs begin to divide, they look like early embryos and form blastocysts with stem cells inside. A young woman with Type 1 diabetes could donate her eggs, which could be activated in the laboratory without being fertilized. Her own stem cells, gathered when the eggs develop to the blastocyst stage, could be used to treat her Type 1 diabetes.
Stem cells developed from an unfertilized monkey egg that went through parthenogenesis are being used to treat Parkinson's disease in monkeys. This line has proven to be as robust as stem cells from human eggs.
One of the reasons this line of research has not been pursued is that in the Dickey Amendment that was put in place by Congress in 1996, parthenogenesis was specifically included with the rest of embryo research, so scientists cannot get federal funding to do work even on unfertilized human eggs. 
Many scientists also believe that the embryo is not actually a human being until two weeks after when a simple nervous system is found.