Germ cells are specialized cells which are involved in reproduction. The most well known examples of this type of cell are gametes, the sperm and eggs which come together to create a zygote which can develop into a fetus. In addition to gametes, a number of other cells involved in reproduction are also germ cells, including gonocytes, the cells which regulate the production of eggs and sperm.
All germ cells carry the germline, the genetic material which an organism can pass on to its offspring. In humans, these cells are haploid, meaning that they carry only half the number of chromosomes necessary to create an organism. When germ cells from two different people meet, their haploid genetic material combines to create diploid cells which can replicate themselves through cell division, ultimately building a baby.
Cells which do not carry the germline of an organism are called somatic cells. The bulk of the cells in your body are somatic cells. Somatic cells are diploid, containing all of the information needed to make an organism, and many of them have special tasks. Flaws in somatic cells such as cancers cannot be passed on to offspring, because somatic cells are not involved in reproduction.
In addition to being haploid, germ cells are technically immortal. They are capable of replicating themselves infinitely, unlike somatic cells, which can only duplicate themselves a limited number of times before they start to mutate or division simply fails. However, research has shown that although germ cells are technically immortal, they are prone to errors in the duplication process as they age, which explains why children born to older people are more prone to genetic defects caused by problems with replication.
These cells are involved at every step of the reproductive process. They generate the gametes which have the potential to join with other gametes to create a fertilized egg, and they regulate the cell duplication which allows a fertilized egg to turn into a fetus and eventually a child. Many researchers are very interested in germ cells because they have some potential applications in research and medical treatment. However, germ cell research is sometimes controversial because the cells are often closely linked with fetuses, a hot topic in many societies due to beliefs about when, precisely, life begins for humans.