A gestational carrier is a woman who carries a pregnancy for another woman. She does not provide a genetic contribution (ovum) to the pregnancy but provides strictly a pregnancy carrier service ("a rented womb") and hands the baby over to the biologic mother at the conclusion of the pregnancy.
A gestational carrier service is a form of surrogate motherhood, however there is a tendency now to limit the term 'surrogacy' to those services where both ovum donation and gestational carrier serves are done together. A famous situation here is the Baby M case that exposed certain legal and ethical risks. Egg donation itself is another example of third party reproduction.
Gestational carrier services may be needed in women who have normal ovarian function but have no uterus, be it the result of a congenital defect (i.e. Mullerian agenesis) or a hysterectomy at young age. Also, in some women, the uterus may be damaged from scarring (Asherman's Syndrome) or leiomyoma.
Women who provide such services may be relatives, friends, or peoples specifically recruited through agencies who do this for a payment, but also altruistically. Careful screening needs to assure their health. generally it is advisable to secure the services through legal documentation. The gestational carrier incures the potential obstetrical risks.