Ovaries are a part of the female mammalian reproductive system. They contain many small follicles which produce the eggs which will merge with the sperm to make a embryo. Also they produce various hormones controlling amongst many other things the menstruation cycle.
Normally, a female will have two ovaries, each containing a fixed number of eggs. In humans, an egg launched from an ovary has to traverse a slight space before entering the fallopian tube and moving gradually down to the uterus where it implants itself into the lining and develops as the pregnancy continues. If the egg settles into the fallopian tube instead of the uterus an ectopic pregnancy will result. If the egg fails to release from the follicle in the ovary an ovarian cyst may form. These cysts are a major sign of a disease called polycystic ovary syndrome.
An ovary is also the female part of a flower. It has a long style and surrounds the ovule. When pollenated, the ovary will grow into the fruit. Some wind pollinated flowers have much reduced and modified ovaries.