A teratoma is a tumor derived from pluripotent stem cells. The word teratoma is from the Greek meaning monstrous tumor. Teratomas are derived from germ cells, sperm and ova and thus can form tissue from endoderm, mesoderm and ectoderm. Thus these tumors can contain hair, bone and even teeth. Rarely, a more developed organ such as an eyeball or hand can be formed. Some forms of teratomas, Fetus in fetu and fetiform teratoma, can even resemble a malformed fetus. Pictures of these types of teratomas can be found here and can be quite shocking.
Here is a link to ovarian teratomas. There are two different kinds of ovarian teratomas. Mature teratomas which are benign and immature teratomas that are cancerous. A short review on teratomas can also be found here.