Muslims are, as a general rule, clean people. The rituals of wudu and ghusl see to that.
Ablution is a common element amongst Semitic religions. Judaism has mikveh, and Christianity has baptism. In Islam it serves as a method of purification before daily prayers, after menstruation, sexual intercourse, and before burial. One ablution, wudu, is for the thousand daily things that impurify us. This is undertaken before daily prayers and requires only a partial ablution. The specific process, requirements, and impurifying acts that require wudu to cleanse varies from sect to sect and school to school, but they all share a desire to purify oneself before making salat, the five daily prayers required by the Quran.
Ghusl, though. Ghusl finds its Quranic basis in the surah al-Nisa, of which a translation reads:
O ye who believe! Draw not near unto prayer when ye are drunken, till ye know that which ye utter, nor when ye are polluted, save when journeying upon the road, till ye have bathed. And if ye be ill, or on a journey, or one of you cometh from the closet, or ye have touched women, and ye find not water, then go to high clean soil and rub your faces and your hands (therewith). Lo! Allah is Benign, Forgiving.
Ghusl is a full body ablution. Again, the specifics vary from sect to sect and school to school, but the unifying principle is centered around purifying oneself to be open to Allah and to know what you are praying. Unclean water cannot be used, and the whole body has to be cleaned. Ghusl is undertaken after sex, after menstruation, after touching a dead body, and before one is buried.
That one last cleansing was something that was robbed from Stephon Clark.