Wudu is a ritual ablution central to a Muslim's worship of Allah the Most High. We perform it several times a day to be able to stand for prayer. Thus, wudu is often called the "key" to prayer.
The following actions must not be done without wudu:
Ritual prayer (namaz).
Touching the Quran.
Going around the Holy Ka'ba (Tawaf).
Performing ritual prostrations such as the sajda of shukr, sajda of tilawah, etc.
Wudu is nullified by several things. The most common are the following three:
Anything that exits from the private openings (both rear and front), including gases. Since gases are included, passing wind also nullifies one's wudu, and so does any amount of filth that exits and appears at the tip or edge of the openings.
Sleep, while lying down or leaning onto something. However, slight slumber or drowsiness while in half-conscious state does not nullify wudu.
Blood or pus that flows from anywhere in the body. Flowing means going beyond the place of exit, so if blood, for example, just surfaces without flowing, it does not nullify the wudu.
Other things that nullify wudu are these four:
Vomiting, such that one cannot withhold it.
Loss of consciousness, insanity or heavy drunkenness.
Laughing out loud, while performing a ritual prayer.
Direct contact between the aroused private parts of a couple. However, if penetration occurs, then ghusl becomes necessary.
Clean vaginal discharge does not invalidate wudu.
Wudu has four obligatory actions:
Washing the face. The face is from the top of the forehead to the bottom of the chin, and its width is from one earlobe to the other.
Washing the two hands and arms, up to and including the elbows.
Wiping (at least) a quarter of the head. The head is the area above the ears where hair normally grows.
Washing the two feet, up to and including the ankles.
When washing the face, arms and feet the water has to cover all the demarcated areas, without leaving a dry place. The difference between washing and wiping is that washing must have water dripping, with the minimum of two drops.
Thick beard: for someone with a really thick beard it is enough to wash the beard's exterior above the face line.
Substances preventing water: all substances (such as nail polish) blocking water from reaching the nails or skin of the parts being washed have to be removed before performing the wudu.
Wounds on the body: If one has wounds, it is sufficient to wipe the wounds or the bandage covering them, and wash what is possible. If the majority of one's limbs are injured, then he performs tayammum instead of wudu.
Not having ghusl: if one is in state of major ritual impurity, then he has to perform ghusl instead. Since ghusl involves washing one's entire body, there is no need to perform wudu separate from ghusl.
To observe the main sunnahs of wudu, one should perform it in the following manner:
Make an intention to perform wudu.
Say Bismillahir-Rahmanir-Raheem. When in a bathroom, the words should be said silently.
Wash the wrists. Start from fingers, wash between them. If there is a ring, it should be move such that water reaches the skin.
Use a toothstick (miswak). In the absence of a toothstick, the thumb and index finger of the right hand can be used.
Rinse the entire mouth and the soft part of the nose three times.
Wash the face three times.
Run wet fingers through the beard.
Wash both hands up to and including the elbows three times starting from the right hand.
Wipe the entire head once, and ears with the same water.
Wash both feet up to and including the ankles three times starting from the right foot. Use fingers to wash between the toes.
Continuity and order: one should keep the order of the aforementioned actions and do them one after another without breaks.
Rubbing: one should also rub the body parts while washing them.
Wudu during fasting: when fasting one must be careful to not let water down the throat while rinsing mouth and nose.
Disliked actions: one should not waste too much water, or use too little of it such that washing becomes wiping. Also one should avoid speaking while making wudu with the exception of making the appropriate duas.