Found it. I'll try to explain the details as best I can.

Stem cells, in a nutshell, are cells that haven't specialized yet (unlike blood cells for example) and thus have the capacity to become almost any of the over 200 types of cells in our body. This provides countless possibilities for regenerative medicine because it may provide a solution to replacing cells lost in diseases, possibly including Parkinson's disease, diabetes, chronic heart disease(by replacing diseased cardiac tissue), liver failure and cancer. Simply think of stem cells as possible spare parts.

Embryonic Stem Cells. About five days after fertilization, the fertilized egg develops into a blastocyst, a hollow sphere of outer cells(later becomes the placenta) and a small cluster of about 30 cells called the inner cell mass which later becomes the human embryo. At this stage, the inner cell mass hasn't specialized yet (hence, stem cell) and is removed and cultured, chemical triggers used to develop it into any of the cells composing the human body except for the placenta. This is very controversial because after conception the fertilized egg is considered alive and removing the inner cell mass prohibits the embryo from developing (ie. killing it).

STOP READING!! Take a deep breath, have a glass of water or coffee if you like and maybe a cookie(chocolate for you potgenie and perhaps a round or two of Starcraft for Darko). Get back to the rest when you're sure your brain has stopped pounding in your head. I don't know if they can replace that with stem cells yet and I don't want to be responsible if your dad says "would you tutor my idiot son/daughter?" to your friends.

Embryonic Germ Cells. These are derived from the gonadal ridge of a fetus (left alone, these develop into eggs/sperm, the gonadal ridge becomes the ovaries or testes). Although different from embryonic stem cells, they too can give rise to virtually all cell types making them candidates for developing medical treatments. The controversy surrounding this centers on the source of these cells, namely aborted fetuses or embryos.

Adult Stem Cells. Less controversial than embryonic stem or germ cells, these are unspecialized cells found in specialized tissue such as bone marrow, spinal cord, liver, and the pancreas. Initial research suggested their scope was more limited but later findings in animal studies suggest they may differentiate into tissues other than those from which they came. Stem cells from marrow have already been used to treat leukemia and other blood disorders. These cells are rare and difficult to identify but at least it will not involve the destruction of human embryos.

If you fall asleep halfway through, it's probably because you didn't stop when I told you to. Source: Medical Marvel, Ethical Minefield and Why the Controversy? articles of the November 22, 2002 issue of Awake!

Ok, got it! Thanks, for that, it's easier to know what I'm talking about when debates come up.

Oh, I forgot to mention.

There are problems yet to be hurdled, such as the rejection of foreign tissue by the recipient's immune system (can possibly be worked around by using the patient's own tissues collected in advance if the disease is not genetically based and by cloning to harvest blastocysts) and a risk of tumor formation, particularly teratoma (aka, "monster tumor," click for wiki article) which may comprise a variety of tissues such as skin, hair, muscle, cartilage and bone. (normal growth can run amok when stem cells are severed from the blastocysts, cultured in vitro and later injected into a living creature)

This is possibly the longest sentence I have ever made.

I cannot believe I actually read the whole thing. I took it as a test - I'm studying genes and such next semester and I wanted to see if I can survive it. Well... it was pretty enlightening information... thanks!

I was wondering about the controversy myself. Thanks for searching that up!