Sure thing.

Very, very early on in the development of an embryo - sometime around days 7-10 after the sperm and egg unite - the cells that make up the embryo undergo a process called gastrulation. Prior to this process, there are essentially two types of cells within the embryo: the cells that will become the placenta and other supportive structures that ensure that the embryo gets the nutrients it needs from the mother (the "trophoblast," "blast" roughly meaning immature and "tropho" roughly meaning food/nutrients), and the cells that will become the actual human organism over the next several months (the "embryoblast"). The cells in the embryoblast are more or less homogenous - they have no determined fate, and they retain their pluripotency, or their ability to become a large variety of different cell types. Remember that every cell in your body more or less has the same genetic material; a primitive, pluripotent cell becomes a specific cell type by turning off and on certain genes, causing the cell to produce proteins, enzymes, etc. that change both its shape and what it is capable of doing. This is the process of cellular differentiation. The fully developed human that you see essentially came from two pluripotent cells. By extension, pregnancy is essentially the process by which those two cells differentiate into the billions of cells and tens to hundreds of different cell types necessary for the body to work.

Back to gastrulation. Gastrulation is a process when the cells of the embryoblast develop into one of three basic cell types from which the rest of the organism will be produced. The three types, also known as "germ cell layers," are the ectoderm (which will largely become cells that line the outside of the body as well as the central nervous system), the mesoderm (which will largely become connective tissue - things like bones, muscles, etc.), and the endoderm (which is largely everything else, most importantly most of the major organs). The names give you a clue here: "ecto" means out, "meso" means middle, and "endo" means inner. There are exceptions to these general rules, of course, but they're a good way of thinking about how these three layers work for our purposes.

In a craniopharyngioma, a small portion of tissue called Rathke's pouch fails to be destroyed during development. This piece of tissue comes from that ectoderm germ cell line, and because this is a primitive piece of tissue, the cells that make it up are relatively pluripotent. This is how you can get something bizarre like teeth growing in a solid tumor: the tumor itself is actually a growth of these immature, somewhat pluripotent ectoderm cells (rather than, say, fully mature teeth cells). Because the processes that cause these immature cells to develop into functional parts of the body are screwed up in tumors like this, some of the cells become remain immature and pluripotent (likely constituting the bulk of the tumor) while others will develop into more recognizable structures. An even more impressive example of this bizarre messed up development is a type of ovarian cyst called a dermoid cyst or teratoma; these tumors/cysts contain cells from all three germ cell lines, so in one tumor you can have a crazy combination of things - teeth, hair, bone, cartilage, skin, and portions of organs like the thyroid, blood vessels, etc. - all in the very odd location of the ovary!

Hopefully that helps.