Firstly: Defining Ablution
Wudu' linguistically: comes from wada'ah, which means beauty, splendour, and cleanliness.
Morphological variances and their meanings: wudu' is the process of ablution, wadu' is the water prepared for it, and mida'ah is the place where it takes place.
Wudu' technically: worshipping Allah, exalted, by washing specific parts of the body in a specific way.  Ibn `Uthaymin said: “If it is said that this definition is insufficient since body parts does not explicitly include the head, then the answer is that this is simply ruling by majority.” Al-Sharh al-Mumti` (183/1)
Secondly: Virtues of Ablution
a) Ablution is half of faith
b) Ablution is an expiation of sins
c) Preserving one's ablution  That is, remaining in a state of purity. is a sign of faith
d) Ablution is one of the reasons for entering Paradise and being adorned by its adornments.
Thirdly: When it is Legislated to Perform
1. Ablution for the call to prayer
It is mustahabb to perform ablution before making the call to prayer.  This will be discussed in the book of prayer.
2. Ablution for prayer
- The ruling of ablution for prayer
Purity from hadath is a condition for the validity of prayer.  This will be discussed in the book of prayer.
- Renewing the ablution for every prayer
It is sunnah  There is a consensus among the scholars that it is permissible to pray more than one prayer with a single ablution. From those who quoted this consensus are Ibn Taymiyyah and others. to perform ablution for every prayer, and this is by agreement of the four schools of jurisprudence.
- Ablution for the funeral
Purity from hadath is a condition for the validity of the funeral prayer.  This will be discussed in the book of prayer.
- Ablution for the prostration of recitation
The scholars have differed over the necessity for purity for the prostration of recitation according to two views: that it is a condition, and that it is not.  This will be discussed in the book of prayer.
3. Ablution for circumambulation
Scholars are consensually agreed upon purity for circumambulation being legislated, but differed over its necessity.  This will be discussed in the book of pilgrimage.
4. Ablution for reciting the Qur'an
It is permissible for the one in a state of minor hadath to recite the Qur'an without touching the mushaf, even though it is preferred that he performs ablution first. Consensus has been quoted over the permissibility of reciting the Qur'an in a state of minor hadath by: Ibn `Abd al-Barr, al-Qadi `Iyad, al-Nawawi, and Ibn Taymiyyah,
5. Ablution for touching the mushaf
It is haram to touch the mushaf without ablution, and this is by agreement of the four schools of jurisprudence.
6. Children touching the mushaf without purity
It is permissible for young children to touch the mushaf for learning and memorisation even if they are not in a state of purity.  Though it is preferred that they are commanded to perform ablution so that they touch it in a state of purity. The majority have explicitly mentioned this: Hanafis, Malikis, Shafi`is, and a position among the Hanbalis.
7. Ablution when about to sleep
It is sunnah to perform ablution for sleep, and this is by agreement of the four schools of jurisprudence.
8. Ablution for one who is junub  In a state of janabah (major impurity) as previously mentioned. when about to eat, drink, and sleep
It is mustahabb for one who is junub to perform ablution when they are to eat, drink, or sleep. This is the position of the majority: Malikis, Shafi`is, Hanbalis, and the choice of Ibn Hazm.
9. Ablution for one to re-engage in intercourse
It is mustahabb for the one who is junub to perform ablution if he intends to re-engage in sexual intercourse, and this is by agreement of the four schools of jurisprudence. It is also the position of the majority of the Companions and those who followed them, as well as the majority of scholars.