What exactly is tamoxifen therapy?

Anti-estrogen. Tamoxifen is a molecule that can attach to estrogen receptors ( sticky hands on cells that like estrogen). If estrogen (female hormone) binds these receptors it typically causes growth/ turn cell on. Tamoxifen mimics estrogen but can have the opposite effect, hence anti-esteogen. Tamoxifen by attachment can turn cells off (in some breast cells) or turn cell on( in uterus).
Chemotherapy. Tamoxifen is an "anti-estrogen" pill used to treat certain breast cancers. It is typically prescribed for 5 years. While it does not cause typical side-effects seen with traditional chemotherapy (hair loss, nausea), it is a very effective chemotherapeutic drug for estrogen-positive breast cancers. Like all medications, side effects are common, and the medication is not suitable for all patients.