These cells are like a master cell in a way. Although they cannot make a human life they do have the ability to form most types of cells. What this means is that if these cells are taken at this stage they can be made into almost any type of cell that is needed. These cells are not what are called totipotent cells which are they type that would be needed for human cloning. .
People tend to disagree with how the cells are attained which is another cause of opposition. One method, which I have mentioned before, is taking the cells from a human embryo . This method would not infringe on the right to life of an embryo if the embryo was already going to be destroyed. Another method is by taking the cells from the fetal tissue of a terminated pregnancy. In this method a life would not be taken because of the removal of the stem cells either. These methods are the most ethical from my point of view as where there are others that are more controversial, but they are not used as much.
Stem cells can be used to fix many types of diseases, conditions, and disabilities. These include problems like Parkinson's and Alzheimer's diseases. Other examples of conditions that could be treated are a spinal cord injuries, stroke, burns, heart disease, diabetes, osteoarthritis and rheumatoid arthritis. With the advances that are possible with stem cell research there is almost no area of medicine that would not be touched.
To show how exactly stem cells would be used I found an article from the National Institute of Health explaining how stem cells would be utilized to help someone with chronic heart disease. Transplanting healthy muscle cells into the heart of someone with this disease could provide new hope for someone with a heart that can no long pump sufficiently. The plan is to develop heart muscle cells from human pluripotent stem cells and transplant them into the failing heart muscle in order to increase the function of the failing heart.