Ablution (Wudu')

Prior to performing the daily prayers, and as a recommended or obligatory prerequisite to other acts of worship, Muslims must purify themselves – this is usually done with water. The minor form of this purification with water is called Wudu, while the major form is called Ghusl. If water is not available, the purification can be performed with clean earth or soil, and is called Tayammum.

Performing Wudu'

"O Believers!  When you prepare for prayers, wash your faces and your hands up to the elbows, and wipe your heads, and your feet to the ankles … and [if you] do not find water then betake yourselves to clean earth and wipe your faces and your hands with it.”

(Qur'an, 5: 6)

In Wudu', it is obligatory to wash the face and hands, and to wipe the front portion of the head and the upper part of two feet.
Washing the face:The length of the face should be washed from the upper part of the forehead, where hairs grow up to the farthest end of the chin, and its breadth should be washed to the part covered between the thumb and the middle finger. If even a small part of this area is left out, Wudu will be void. Thus, in order to ensure that the prescribed part has been fully washed, one should also wash a bit of the adjacent parts.
The face and hands should be washed from above downwards.

Washing the arms:After washing the face, one should first wash the right hand and then the left hand, from the elbows to the tips of the fingers. In order to ensure that each elbow has been washed thoroughly, one should include some portion above the elbow in washing.
While performing Wudu, it is obligatory to wash the face and the hands once, and it is recommended to wash them twice. Washing them three or more times is haraam.

Wiping the head

:After washing both the hands, one performing Wudu should wipe the front part of his head with the wetness which is in his hand; the recommended precaution is that he should wipe it with the palm of his right hand, from the upper part, downwards. The part on which wiping should be performed is one fourth frontal part of the head. It is sufficient to wipe as much at any place in this part of the head, although the recommended precaution is that the length should be equal to one finger, and its breadth should be equal to three joined fingers. It is not necessary that the wiping of the head should be performed on its skin. It is also in order if a man wipes the hair on the front of his head.

After wiping the head, one should wipe with the moisture present in one's hands, one's feet from any toe of the foot up to the joint. As a recommended precaution, the right foot should be wiped with the right hand, and the left foot with the left hand.

Taken from:

http://al-islam.org