Estrogen is not one hormone, it is the name of a group of hormones.

There are three principle forms of estrogen found in the human body estrone, estradiol and estriol, also known as E1, E2 and E3 respectively.

There is also a group of compounds called phytoestrogens, generally found in food, which can have "estrogen like" effects in the body.
Estradiol (E2) is the primary estrogen produced by the ovaries.
Estrone (E1) is formed from estradiol. It is a weak estrogen and is the most abundant estrogen found in the body after menopause.
Estriol (E3) is produced in large amounts during pregnancy and is a breakdown product of estradiol. Estriol is also a weak estrogen and may have anti-cancer effects. Before menopause estradiol is the predominant estrogen. After menopause estradiol levels drop more than estrone so that now estrone is the predominant estrogen.

Conjugated equine estrogen has been the most commonly prescribed estrogen supplement in the U.S.
Conjugated estrogens are derived from pregnant mare's urine. They must be converted by the body into active estrogens.

Estradiol (E2) is now widely prescribed in the form of skin patches, tablets and creams and is gaining on popularity.
A combination of E1,E2 and E3 called Triple Estrogen has been available for many years. It's proponents claim it is the most natural way to take estrogen. Triple estrogen is difficult to get.