Estrogen is a group of hormones that are key to the development and regulation of the female reproductive system and secondary sex characteristics. There are several types of estrogen, including estrone, estradiol, and estriol. Estrogen plays a crucial role in regulating menstrual cycles, maintaining bone density, and supporting the cardiovascular system. During menopause, estrogen levels decrease, leading to a variety of symptoms such as hot flashes, night sweats, and mood swings.