Anastrozole (Arimidex) manages by preventing the aromatase enzyme, the principal enzyme for the alteration of testosterone to estrogen. Used in PCT cycles by bodybuilders and during (heavy) steroids cycles.
A steroid that is modified by this enzyme is known as an aromatizing steroid, and such steroids can cause estrogen build-up. It has several potential side-effects such as fluid retention, excess fat gain and enables remember gynocomastia (the progress of breast cells in men).
Often times throughout a cycle most would want to enable some estrogen, since it closely promotes power and profits as well (boosts GH, updates the androgen receptor, increases glucose usage). These folks will generally select for an estrogen receptor antagonist such as Nolvadex (tamoxifen) or Clomid (Clomiphene). The products do not stop the forming of estrogen, but stop the estrogen from exerting its results by competitively taking on the receptors because of this hormone. This enables them to avoid any nagging problems dead in their tracks, acting extremely fast, but after discontinuation allowing for immediate influx of estrogen again as well.