In Islam, ablution, or Wudhoo', is to wash one’s face, hands, arms, head and feet with water. Wudu or Wazu is the Islamic procedure for washing parts of the body using water often in preparation for formal prayers (salah). Wudu literally means cleanliness and in Shari'ah (Islamic Law) it is washing of the three specified parts of the body and wiping of the head. Without Wudu, one cannot perform Salah.
As Allah (Glory be to Him) says in Qur’an: “O you who believe! when you prepare for prayer, wash your faces, and your hands (and arms) to the elbows; Rub your heads (with water); and (wash) your feet to the ankles.” Qur’an .Surah Ma’idah 5:6 
Abu Hurairah (R) reported that the Messenger of Allah (May Allah honour Him and grant Him peace), said, "When the Muslim - or believing - slave does wudu' and washes his face, every wrong thing at which his eyes have looked leaves with the water - or with the last drop of water. When he washes his hands, every wrong thing which his hands have touched leaves with the water - or with the last drop of water. When he washes his feet every wrong thing to which his feet have walked leaves with the water or with the last drop of water, until he emerges cleansed of sins." Sahih Al Muslim 148 and 475, Muwttah Malik 32.
'Umar ibn al-Khattab (R) reported that the Prophet Muhammad (May Allah honour Him and Grant Him peace), said, "No one among you does wudu' and does wudu' thoroughly or adequately - and then testifies, 'There is no god but Allah (Glory be to Him) Alone with no partner and I testify that Muhammad is His slave and Messenger' without the eight doors of the Garden being opened to him so that he can enter by whichever of them he wishes." Tirmidhi Vol 1:55.
'Usman ibn 'Affan (R) reported that the Messenger of Allah (May Allah honour Him and Grant Him peace) , said, "Anyone who does wudu' and does it well, his mistakes leave his body, even from under his nails." Sahih Al Muslim 476 
One should perform wudhu before the prayer, for the Prophet said: "Prayer is not accepted without purification….” (Muslim #224)
“I saw ‘Uthmaan (R) perform wudhu. He poured water on his hands thrice, then he rinsed his mouth and nose, washed his face thrice, washed his right hand up to his elbow thrice, washed his left hand up to his elbow thrice, wiped over his head once, washed his right foot thrice, and then his left foot thrice. He then said, ‘I saw the Messenger of Allah perform wudhu like this, and he said: ‘Whoever performs wudhu like my wudhu, and then prays two Rakaat not thinking about anything else, Allah will forgive him all his previous sins.’ (Bukhari #1832) 
1) Wudu is a Fard (Obligatory) for performing Salaah
2) For touching the Qur'an (according to some scholars it is obligatory and according to some other scholars it is Mustahab).
4) Mustahabb (Desirable) - Wudu is recommended before going to sleep. 
There are four compulsory aspects of performing Wudu, it is important that any objects that hinder water from reaching the skin must be removed so that the skin may be washed properly.
1) To wash the face once with purifying water. Length wise the face extends from the hairline of the forehead to below the chin and breadth wise from one earlobe to the other.
2) To wash the hands once up to and including the elbows.
3) Wiping a quarter of the head once with wet hands.
4) Washing the feet once up to and including the ankles. 
The following actions will cause the Wudu to be broken:
1) Excretions of any form like urine, faeces or wind.
2) Direct contact of hands with private parts without clothes in between.
3) Deep sleep with which he is in unconscious state of mind and one is unknown of the fact that faeces or wind comes out of his private parts.
4) Unconsciousness due to madness or swoon or intoxication.
5) Eating camel's meat
6) Apostasy  
According to Dr. MAGOMEDOV, Chinese medicine says that there are more than 700 BASes, and sixty-six of them have quick reflex therapy effects and are named the drastic (or aggression or antique or prime-elements) spots.
Out of these sixty-six spots, sixty-one of them are located in zones required for ablution while the other five are located between the ankle and knee.
Thus, ablution becomes a kind of treatment complex, which includes the hydromassage of the BAS, their thermal and physical stimulation.
Dr. Magomedov said that his studies were triggered by his solemn belief that the five-time-a-day Muslim prayers were bound to have not only an indisputable spiritual effect but were also bound to have a purely physical healing effect as well.
Ablution, helps prevent skin cancer. This is his explanation: the areas that are washed during ablution are the parts of the body that are most prone to be exposed to pollution, whether it is pollution from the internal secretions of the body on to the skin surface, such as sweat, or whether it is external. Ablution, removes this pollution five times a day, and hence maintains a clean outside layer of skin, which in turn helps the cells underneath to function properly.
Also, washing with water helps invigorate the ends of the blood vessels, as well as the nerves and glands that are near the skin surface, and hence helps them perform their functions efficiently. [Salem, Mukhtar, Prayers: A sport for the soul and body, CAIRO, The Arab Modern Center (1990), pg. 52.] 
1. A person cannot find water, or the amount available is insufficient.
2. A person has no access to water.
3. A person is saving the water for later use.
4. A person is sick and cannot use water.
5. The use of water is likely to do harm, cause or worsen an illness, or delay the cure.