(Newbie) What is HCG????

Is this a scientific name, a brand name or what? There are som many acronyms running around this site I can't keep up. Thanks for answering my surely repetitive newbie question.

http://www.steroid.com/66.php

Another.....

http://forums.steroid.com/educational-threads/640-human-chorionic-gonadotropin-hcg.html