embryonic stem cellsWatch this thread
I don't know if you know what a blastocyst is? Basically after you form a zygote, it starts dividing, and it forms a structure that is basically a ball of cells surrounding a cavity, but at a certain point there is a group of cells together, called the inner cell mass. Embryonic stem cells are cells from that inner cell mass, and they can give rise to all the structures you need to make an embryo/ baby. A small part of it participates in making the placenta as well.
They are the cells that make up the lining of the blastocyst and they give rise to membranes like the largest part of the placenta, the amnion that surrounds the embryo and something called the yolk sac that provides nutrition for the baby before the placenta becomes functional.