Ovaries
As HealthSquare explains, the ovaries have a critical role in the female reproductive system because they contain the eggs. At puberty, the average female has 300,000 eggs cells in the ovary that are surrounded by sac-like structures called follicles. Most of these egg cells will never become a fully mature egg, however. Each month, approximately 20 follicles start to mature in response to rising levels of two hormones called follicle stimulating hormone and luteinizing hormone. The follicle that matures the fastest releases a mature ova (egg) and the other follicles stay dormant. The release of an egg is called ovulation.
Fallopian Tubes and Vagina
The fallopian tubes are structures that connect the ovaries to the rest of the reproductive system and are also the place where fertilization occurs. As KidsHealth explains, sperm enter the female reproductive system through the vagina. In order to get to the fallopian tubes they have to pass through the cervix, which is a structure at the back of the vagina. Once they are past the cervix the sperm can travel up the uterus to the fallopian tubes to the egg in order to fertilize it.
Uterus
The uterus is the portion of the female reproductive system where a fetus develops. After ovulation the ovaries secrete a hormone called progesterone which thickens the lining of the uterus (also known as the endometrium). If an egg has been fertilized, it travels down the fallopian tube and implants itself into the endometrium. If there is no fertilized egg, the endometrium will eventually dry up and slough off. This results in some bleeding which comes out of the vagina in a process known as menstruation.
