Ghusl (Arabic: غسل‎ Ġusl , IPA: [ˈɣʊsl]) is an Arabic term referring to the full body ritual washing ablution required, if the adult loses the state of body cleanness, in Islam for adults prior to ablution (wudu وضوء) for various rituals and prayers. Ghusl is mandatory for any adult Muslim after having sexual intercourse, orgasmic discharge (e.g. semen),[1][2] completion of the menstrual cycle,[3][4] giving birth, and death by natural causes.[5]

Islam also recommends (i.e. it is mustahab) the performance of the full ablution before the Friday sermon prayer[6][7] and Eid[8] prayers, before entering the ehram, in preparation for hajj,[9] after having lost consciousness,[9] and after formally converting to Islam. Shia Muslims also perform the ablution before Namaz-e-tawbah.

Ghusl should not be confused with wudu, a partial ablution, that Muslims perform before prayer salat.
http://en.wikipedia.org/wiki/Ghusl