Wikipedia also has an article on this topic.
The estrogen receptor (ER) is the receptor of estrogens like estradiol. Agonists of the ER include estradiol, ethinylestradiol, and conjugated estrogens. Partial agonists of the ER, or selective estrogen receptor modulators, include tamoxifen, clomifene, and raloxifene. Antagonists of the ER include fulvestrant.
There are two different forms of the estrogen receptor, ERα and ERβ. ERβ may have anti-proliferative effects that oppose the effects of ERα.