A human embryo is the first stage of human development.
It is developed by cell multiplication of a zygote, formed by the fusion of the male gamete (spermatozoon or "sperm") and female gamete (ovum or "egg"). This is called fertilization and usually occurs in the proximal portion of the female reproductive tract. After fertilization, an embryo must implant in the lining of the uterus. If it does not, several problems can occur. The pregnancy can be lost via miscarriage. This often occurs without the knowledge of the mother. A tubal pregnancy can occur. This is a medical emergency requiring prompt intervention.
There are several controversial issues involving human embryos.
1. IVF, that is, In Vitro (in glass) Fertilization: Traditionally, fertilization occurs via insemination of a female by a male during sexual intercourse. However, reproductive science has created many advances in fertilization, allowing infertile couples to have their own biological children. This embryo is then surgically implanted in the mother. The first so-called "test tube baby" was born in 1978 and is now a healthy female adult. There has been a great deal of controversy about this practice, and there are many interesting moral/religious viewpoints on the topic. Many see IVF as a way for couples to help fulfill God's commandment to be fruitful and multiply, others see it as science invading the territory of natural marital reproduction.
2. Abortion: abortion interrupts the development of an embryo into a living baby separate from its mother. This issue is addressed in depth elsewhere.
3. Stem cells: also addressed extensively elsewhere.