Arimidex (anastrozole) is what is classified as an aromatase inhibitor. It's original designed use was to treat breast cancer in women by lowering estrogen. It was eventually discovered that arimidex could benefit bodybuilders by blocking estrogen formation or lowering already existing levels.
As I talked about in the Nolvadex 101 article, the biggest difference between an aromatase inhibitor such as arimidex and a selective estrogen receptor modulator like nolvadex is that arimidex blunts estrogen formation and nolvadex simply control it by not allowing any receptor binding.
However, this doesn't mean that already circulating levels of estrogen are not present just because nolvadex is included in the drug regimen.
See also: How to Keep Estrogen Under Control
Arimidex works by reducing the aromatase enzyme which is what is the cause of elevated estrogen levels and a byproduct of drugs that can aromatize while on cycle.
Arimidex is superior to nolvadex in almost every possible way with the exception of stimulating luteinizing hormone during cycle recovery (studies are showing that nolvadex is better at this than AI's, but this is a double edged sword because AI's are better at combating estrogen rebound)