Tayammum (or 'dry ablution') is a method of cleansing oneself using clean sand. It serves as a replacement for both Wudu(partial ablution) and Ghusl (complete ablution) but only under special circumstances. The Holy Quran says:
"And if you are ill or on a journey or one of you comes from the place of releiving himself (or herself) or you have contacted women (or men i.e. had sexual intercourse) and find no water, then seek clean earth and wipe over your faces and your hands (with it)." Al Quran, [4:43]