We've heard the term time and time again the past several years ... but what exactly is stem cell research?

It's an avenue of research that garners friends and foes, and even celebrities are weighing in. Scientists say the potential is nearly boundless, while critics say it's morally appalling.

But first, let's be clear: Not all stem cell research prompts this controversy. Research on adult stem cells is widely accepted and supported. It's the stem cell research conducted on human embryos that draws fire -- and it may also hold the most potential.

Stem cells are "blank" cells that are capable of developing into many of the different kinds of cells found in humans. They're also capable of dividing and renewing themselves over and over, while staying "blank."

Researchers believe that stem cells hold a lot of promise for medical therapies such as growing replacement cells or whole replacement organs, as well as patching organs that don't work properly -- like helping a diabetic person's pancreas produce insulin.

Adult stem cells are found in very small numbers in different human tissues and the blood of babies' umbilical cords. Research began on adult stem cells more than 40 years ago, when one stem cell type -- hematopoietic, or blood, stem cells -- were first discovered in bone marrow. Since then, stem cells have been found in several other tissues, and adult blood-forming stem cells from bone marrow have been used for successful transplants for more than 30 years as a treatment for blood cell cancers such as leukemia, lymphoma and multiple myeloma, according to the National Institutes of Health.

Research with adult stem cells is ongoing. During the past decade, scientists have discovered adult stem cells in tissues that were previously thought not to contain them, such as the brain.

Embryonic stem cells are found only in human embryos a few days after fertilization. Embryos for research are created in a laboratory, through in-vitro fertilization when a couple is trying to have a child. Since removing a woman's egg is an invasive procedure that carries risks, doctors remove more eggs than are needed for one child to ensure a quality embryo can later be implanted in the uterus so there is a greater chance for a successful pregnancy. But since the eggs can't survive in a frozen state, they must be fertilized -- thus creating embryos that are then frozen for later use. If the couple decides not to have more children, the embryos are discarded. Now, couples have the option of donating these embryos for use in research instead.

If the embryos are donated for research, stem cells are extracted in a process that destroys the embryo. This is the basis for most of the controversy surrounding the subject.

Human embryonic stem cells were first isolated in 1998, and since then, scientists around the world have been advancing the research. To date, no human therapies have been developed by use of embryonic stem cells, but researchers maintain that they have the potential to treat or cure a variety of ailments -- and even grow entire organs for transplants.