An anti-estrogen is a substance that blocks the production or utilization of estrogens (the family of hormones that promote the development and maintenance of female sex characteristics) or inhibits their effects.

Hot flashes, osteoporosis, breast atrophy, and vaginal dryness can be side effects of anti-estrogens.

Aromatase inhibitors could be considered anti-estrogens by some definitions, but they reduce the production of estrogen, while the term "anti-estrogen" is usually reserved for agents reducing the response to estrogen.

Selective estrogen receptor modulators (SERMs) are a class of drugs that act on the estrogen receptor (ER). A characteristic that distinguishes these substances from pure ER agonists and antagonists (that is, full agonists and silent antagonists) is that their action is different in various tissues, thereby granting the possibility to selectively inhibit or stimulate estrogen-like action in various tissues.
SERMs are competitive partial agonists of the ER. Different tissues have different degrees of sensitivity to and activity of endogenous estrogens, so SERMs produce estrogenic or anti-estrogenic effects depending on the specific tissue in question as well as the percentage of intrinsic activity (IA) of the SERM. An example of a SERM with high IA and thus mostly estrogenic effects is chlorotrianisene, while an example of a SERM with low IA and thus mostly anti-estrogenic effects is ethamoxytriphetol. SERMs like clomifene and tamoxifen are more in the middle in their IA and their balance of estrogenic and anti-estrogenic actions in comparison. Raloxifene is a SERM that is more anti-estrogenic than tamoxifen; both are estrogenic in bone, but raloxifene is anti-estrogenic in the uterus while tamoxifen is estrogenic in this location.

A selective estrogen receptor degrader or downregulator (SERD) is a type of drug that binds to the estrogen receptor (ER) and, in the process of doing so, causes the ER to degrade and thus downregulate. Marketed SERDs include the anti-estrogen (ER silent antagonist) fulvestrant and the selective estrogen receptor modulator (SERM) bazedoxifene.
SERDs are used in the treatment of ER-positive breast cancer, and are considered to be an improvement over previous, less efficacious therapies, like the SERM tamoxifen.

Adapted from Wikipedia, the free encyclopedia. Internet. Accessed on September 25, 2016.