I'm sure you're referring to embryonic stem cells and the controversy involving their use. First, let's see start with what a stem cell is.
Multi cellular organisms have a large number of cells that are differentiated to perform specific tasks. Stem cells have the ability to divide and create more stem cells which can then get converted into any of the other cell types. This serves two broad natural purposes. Embryonic stem cells are present in the human embryo and they help in the formation of all the complex organs of the offspring to be born. Just a few 100 of the same type of cells, are differentiated to form all the cells that make up the human body. Adult stem cells help in the repair and generation of tissue in the body. They are found in bone marrow, adipose tissue and the blood.
Embryonic stem cells are the most potent and can be used to cure many illnesses that do not have a cure in conventional medicine. The controversy behind their use is in the extraction process of the cells from the embryo which leads to its death. This is considered equivalent to killing a living human being by a large number of people.
As an alternative to the use of embryonic stem cells, somatic stem cells and stem cells extracted and preserved from the umbilical cord can be used.