Most people are very familiar with the term "stem cell" even if they don't fully understand what it means. Most people also understand that there is a fair amount of controversy surrounding the use of stem cells in research.
Personally, as a biologist with a decent understanding of stem cells, I feel very uncomfortable using embryonic stem cells for research and potentially therapy. When a new, single-cell human embryo is formed by fertilizing an egg with a sperm, all of its genes are ready for action and can be used. This continues up until the embryo consists of eight cells. After the eight-cell stage, different genes start being turned on and turned off in different cells, often irrevocably. This is good, because this is what allows complex creatures, like humans, have different body parts. Your brain cells have the exact same genes in their cells as your liver cells, but only specific genes are expressed in each organ. And once those pathways have been set, they do not turn back around.
That's why so many scientists see the promise of using stem cells- those cells that can be turned into whatever we want. However, to date, the most useful stem cells are those that come from embryos. This consists of creating a human embryo, the beginning of a human life, and then systematically destroying it for the sake of research. I do not claim to know at what stage of embryological development life "begins," but I know that I am not comfortable with the idea of playing around with the formation and destruction of human life.
This is a difficult stance to hold though, when the possibilities of what stem cells can do for many diseases are looked at. Granted, all stem-cell cure based research is still in very experimental stages- I'm not aware of any tests that have actually advanced to human subjects yet, which means that we don't actually know what stem cells will do when they are used to cure human diseases.
I should know by now that it's foolish to get comfortable in any dogma or paradigm in science, though. Remember how once a cell has been turned into a brain cell or a liver cell, it stays that way for life, and can only ever divide to produce cells like itself, with the same genes turned on and off? It turns out that this may not always be the case. It seems that scientists have taken mature cells of one type and turned them into mature cells of a completely different variety by switching which genes are on and which are off- in a mouse (or what we call in vivo), no petri dish (in vitro) or stem cells required. While it's a little early to start sounding the horns, this could be huge. This could eliminate lots of time, money, and controversy by cutting out the need for stem cells.
I strongly recommend reading the linked article. It's written in a fairly easy to understand way (I think) and it's so fascinating! Kids, science and medicine are changing fast. Hold on to your seats.