Teratomas are germ cell tumors that may arise in a variety of places within the body.  The majority are gonadal (ovary or testes) in origin.  Newborn teratomas of the tailbone (sacrococcygeal teratoma) are covered separately.

Basics
A teratoma is a true tumor or neoplasm composed of multiple tissues of kinds foreign to the parts in which it arises.

Where do they come from? Several theories abound. One suggests wandering Germ Cells are left behind during the migration of embryonic germ cells from yolk sac to gonad during fetal development.

The tumor must (by definition) contain elements derived from more than one embryonic germ layer, must be foreign to the anatomic site and cannot have resulted from metaplasia. It must also show a high degree of disorganization

The tumor may be either: (1) Benign (2) Malignant (3) Benign with Immature Elements (worrisome).

Benign Teratomas contain only recognizable adult mature tissues such as epithelium, bone, cartilage, epithelium

Malignant Teratomas contain a malignant tumor among the benign parts.
Examples include Embryonal carcinoma, Yolk Sac Tumors, endodermal sinus tumor, Choriocarcinoma, Germinomas, seminomas, dysgerminomas, Mixtures of Malignant Cells

Immature Teratomas contain cells that are not malignant, but are fetal and should not be present in a newborn. These immature elements may or may not represent cancer. The decision to give chemotherapy must be individualized. Often blood test are used to help guide the decision.

Read more about Sacrococcygeal Teratomas.