Perimenopause or  the Menopausal Transistion, is the time when your body transitions to menopause. Perimenopause begins at different ages, some women begin noticing changes in their 30's.

Menopause is often a natural decline in reproductive hormones that  usually begins 12 months after the last menstruation. 

Premature Menopause (Early), when menopause occurs before the age of 40. This could happen when the ovaries stop producing hormones and your period stops. Other reasons are due to medical when both ovaries are removed during a hysterectomy (Surgical Menopause). Although uncommon it is possible to experience menopausal symptoms even if one ovary were removed. 

Surgical Menopause/Medically Induced Menopause, occurs because of surgery and not the natural transition of life. Surgical Menopause occurs due to an oophorectomy a surgery that removes the ovaries which is the main source of estrogen for women, when this is performed the body will immediately enter menopause. Medically Induced Menopause, is a term often used to express Surgical Menopause however, medical menopause also refers to an induced menopause due to treatments such as chemotherapy, radiation, hormone therapies such as Zoladex, Lupron which temporarily shut down the ovaries.