Ghusl is a major ablution as opposed to wudu which is a minor ablution. In Islamic laws, ghusl is considered an act of worship; it is an act of purifying oneself from the ritual impurity (najasat) caused by sexual intercourse, discharge of semen or blood, and by touching the dead body. The ritual bath given to a dead Muslim before burial is also known as ghusl.
The ghusl for each of these causes has different names: Purification from the impurity caused by sexual intercourse or discharge of semen is known as ghusl janabat. Purification from the impurity caused by menstruation is known as ghusl hayd. Purification from the impurity caused by irregular bleeding is known as ghusl istihadah. Purification from the impurity caused by post-natal bleeding is known as ghusl nifas.
In Islamic laws, death is also considered a cause of ritual impurity of a Muslim's body. Therefore, a dead Muslim has to be washed ritually before the burial ceremony. Such a ritualistic bath for a dead Muslim is known as ghusl mayyit. Touching a dead body, before.the ritualistic bath, also makes one impure (najis). Purification from this impurity is known as ghusl mass mayyit.
In this chapter we shall explain the method and the general rules of ghusl. In chapter 4, we shall discuss the rules of ghusl janabat. The ghusls related to women have been discussed extensively in my The Ritual Ablutions for Women (Taharatu ' n-Nisa' ).
B. MANNER OF PERFORMING GHUSL
Before explaining the rules of performing the ghusl it is necessary to mention that all the ghusls are performed in the same manner; the difference is only in the niyyat of each ghusl. For example, for purifying oneself from the ritual impurity of sexual intercourse, one has to make the niyyat that `he is doing ghusl janabat'.
Ghusl is a ritual bath; it involves washing of the whole body. There are two methods of performing ghusl. One is known as ghusl tartibi, and the other is known as ghusl irtimasi.
1. Ghusl Tartibi:
"Ghusl tartibi" means an ordinal bath, performed in three stages.
After washing away the najasat (e.g., semen or blood) from the body and after niyyat, the body has to be washed in three stages: First, head down to the neck; then the right side of the body from the shoulder down to the foot; and lastly, the left side of the body.
Each part should be washed thoroughly in such a way that the water reaches the skin. Special care should be taken while washing the head; the hair should be combed (e.g., with your fingers) so that water reached the hairroots. While washing the right side of the body, some part of the left side must be washed too, and also, while washing the left side of the body, some part of the right side must be washed.
2. Ghusl Irtimasi:
"Ghusl irtimasi" means a bath involving immersion of the whole body in the water. It is needless to say that such a ghusl can only be done in a body of water, e.g., a pool, river, lake or sea.
After washing away the semen or blood from the body and after niyyat, the whole body should be completely immersed in the water all at once, not gradually. One has to make sure that the water reaches all parts of the body, including hair and the skin under it.
However, ghusl tartibi is preferred to ghusl irtimasi.
C. RECOMMENDABLE ACTS OF GHUSL
What has been mentioned above are the wajib acts of ghusl; here we shall explain the things which are recommendable (mustahab, sunnat) during the ghusl. These recommendable acts are five:
1. Washing both hands up to the elbows three times before the ghusl.
2. Gargling three times.
3. Wiping the hands on the whole body to ensure that every part has been thoroughly washed.
4. Combing the hair with the fingers to ensure that the water reaches the hair-roots..
5. (For men only) Doing istibra' before ghusl janabat. Istibra', in the present context, means "urinating." The benefit of istibra': If a liquid comes out of one's penis after completing the ghusl, and he doubts whether it is semen or urine, then should he repeat the ghusl or not? If he had done istibra' before the ghusl, then he can assume that the liquid is urine -he will not have to repeat the ghusl; he just has to do wudu for his salat. But, on the other hand, if he had not done istibra' before the ghusl, then he has to assume that it is the remnant of semen-he will have to do the ghusl again.
`Ubaydullah al-Halabi narrates that someone asked Imam Muhammad al-Baqir (as) about a man who performs ghusl and then finds some (doubtful) drops (on his penis) while he had already urinated before performing the ghusl. (That is, should he consider the drops as urine or semen?) The Imam said, "He will just have to do wudu (for his salat). But if he had not passed urine before the ghusl, then he must repeat the ghusl." 
This rule of istibra' applies only to men. Sulayman bin Khalid asked Imam Muhammad al-Baqir (as) about a man who became ritually impure because of sexual intercourse and then performed ghusl without urinating. Then some drops came out of him. The Imam said, "He must repeat the ghusl." Sulayman: "What if similar drops come out of a woman after she has performed ghusl?" The Imam said, "She does not have to repeat the ghusl." Sulayman: "What is the difference between the two?" The Imam said, "(A woman does not have to repeat ghusl janabat) because what comes out of her is certainly from the (remnants of the) discharge of man." 
D. A SUMMARY OF GHUSL
This is a summary of ghusl. The recommendable acts of ghusl are in italics type-face.
1. Remove the najasat (semen, blood) from the body.
3. Wash the hands up to the elbows three time.
4. Gargle three times.
5. Wash the head down to the neck; wipe your hand on the face and neck, and comb the hair with your fingers.
6. Wash the right side of your body from the shoulders down to the feet; include some part of the left side also. While washing, wipe the body with your hand.
7. Wash the left side of your body from the shoulders down to the feet; include some part of the right side also. While washing, wipe the body with your hand.
E. THE CONDITIONS FOR THE VALIDITY OF GHUSL
The validity of ghusl depends on certain conditions which are known as "the conditions for the validity of ghusl". These conditions are ten in number: three conditions are related to the water, four are related to the person and three to the act of ghusl itself.
i. The Water:
1. The water must be mutlaq (unmixed, pure).
2. The water must be tahir (ritually clean).
3. The water must be mubah (lawful). The details of these conditions are same as the conditions of the water of wudu.
ii. The Person:
5. All parts of the body must be clean from the impurity (e.g., semen, blood) before starting the ghusl.
6. Use of water should not be harmful to the person who wants to perform ghusl.
7. The ghusl must be performed by the person himself. (The details are same as in wudu)
iii. The Ghusl
8. The place where ghusl is being performed must be mubah (lawful).
9. The ghusl should be performed either in tartibi manner or in irtimasi manner.
10. All parts of the body must be washed thoroughly as explained above.
F. SOME GENERAL RULES
1. If more than one ghusl become wajib on a person, e.g., janabat, mass mayyit, etc., then one ghusl with the niyyat of all of them will suffice. Zurarah bin A'yun quotes Imam Muhammad al-Baqir (as) as follows: "When you perform ghusl (for example, after dawn), that one ghusl suffices for the (ghusls of) janabat, jum`ah, `Arafah, nahr, halq, sacrifice and ziyarat. When various ghusls become wajib upon you then one ghusl will suffice ...And the same (rule) is for the woman; one ghusl will suffice for her ghusl of janabat, ihram, jum'ah, and her ghusl for hayz and `idd." 
2. All the ghusls, except the ghusl for "medium istihazah," suffices the performer from wudu-provided none of the nawaqiz of wudu' have taken place after the ghusl. So a person who has done ghusl janabat, for example, can pray without doing wudu'. Zurarah quotes Imam Ja'far as-Sadiq (a. s.) about the method of performing ghusl janabat as follows: "...there is no wudu, neither before it nor after it." 
3. If one of the nawaqiz of wudu' (e.g., passing of urine) takes place during ghusl janabat, then one must perform the ghusl again, and in such a case he is also recommended to do wudu' after the ghusl. If one of the nawaqiz of wudu' takes place during the second ghusl, then the ghusl will not be affected; but one has to do wudu after it for salat..
4. If one of the causes which makes ghusl wajib takes place during a ghusl, then there are two possibilities: (a) either the cause is similar to the cause which necessitated the present ghusl, then one has to perform the ghusl again; (b) or cause is dissimilar to the cause of the present ghusl, then he should complete the ghusl and then do another ghusl.
5. Before washing the right side of the body, if one doubts whether or not he has washed the head and the neck, then he should start again from the beginning. But if he doubts after commencing to wash the right side, then he should disregard his doubt. While washing the left side of the body if one doubts whether or not he has washed the right side, then he should wash the right side and then wash the left.
6. Ghuslu ' l-Jabirah:
If one has a bandage on his body, then how should he or she do ghusl? Such a person will do ghuslu ' 1 jabirah. Ghusl jabirah can be done by observing the rules mentioned in wudu'u 'l jabirah: either one lifts the bandage and washes the wound normally, or he washes only around the wound or over the bandage, etc.
. Wasa'il, vol. 1, p. 517.
. Ibid, p. 482.
. Wasa'il, vol. 1, p. 526.
. Wasa'il, vol. 1, p. 515; also see p. 50