Embryonic stem cells are taken from the inner part of a young embryo, also known as a blastocyst. An embryo reaches the blastocyst stage about 4-5 days after fertilization. At that point they contain about 50-150 cells.
Embryonic stem cells can differentiate into the three "germ layers": ectoderm, endoderm and mesoderm. The 220 types of cells in humans are all based on these germ layers. Some scientists, particularly those having pro-choice political views, claim that embryonic stem cells can differentiate better, and divide into greater numbers, than adult stem cells can.
Pro-life conservatives consider taking stem cells from human embryos, as tantamount to abortion, which they believe to be a hideous and evil practice. President Bush was pressured by Liberal Scientists into funding stem cell research[1].