As you know that the Shi`a say that Ali (rah) is the one who should have been the first khalifa and not Abu Bakr (rah) (or Umar (rah) or Uthman (rah)).  They bring some evidences from the Sunni books (Bukhari, Muslim,..) and one of them is the hadeeth of Ghadeer Khumm.

Before we start, we should Say that Ali (rah) is the husband of the daughter of the prophet (peace be upon him), Fatima (rah, the best woman of the people of the Jannah), he is the cousin of the prophet (peace be upon him), and he is the fourth of the rightly-guided khalifahs and his qualities are all well known to us. 

We start by reading the hadeeth from Sahih Muslim Book 031, Number 5920:

Yazid b. Hayyan reported, I went along with Husain b. Sabra and 'Umar b. Muslim to Zaid b. Arqam and, as we sat by his side, Husain said to him: Zaid. you have been able to acquire a great virtue that you saw Allah's Messenger (may peace be upon him) listened to his talk, fought by his side in (different) battles, offered prayer behind me. Zaid, you have in fact earned a great virtue. Zaid, narrate to us what you heard from Allah's Messenger (may peace be upon him). He said: I have grown old and have almost spent my age and I have forgotten some of the things which I remembered in connection with Allah's Messenger (may peace be upon him), so accept whatever I narrate to you, and which I do not narrate do not compel me to do that. He then said: One day Allah's Messenger (may peace be upon him) stood up to deliver sermon at a watering place known as Khumm situated between Mecca and Medina. He praised Allah, extolled Him and delivered the sermon and. exhorted (us) and said: Now to our purpose. O people, I am a human being. I am about to receive a messenger (the angel of death) from my Lord and I, in response to Allah's call, (would bid good-bye to you), but I am leaving among you two weighty things: the one being the Book of Allah in which there is right guidance and light, so hold fast to the Book of Allah and adhere to it. He exhorted (us) (to hold fast) to the Book of Allah and then said: The second are the members of my household I remind you (of your duties) to the members of my family. He (Husain) said to Zaid: Who are the members of his household? Aren't his wives the members of his family? Thereupon he said: His wives are the members of his family (but here) the members of his family are those for whom acceptance of Zakat is forbidden. And he said: Who are they? Thereupon he said: 'Ali and the offspring of 'Ali, 'Aqil and the offspring of 'Aqil and the offspring of Ja'far and the offspring of 'Abbas. Husain said: These are those for whom the acceptance of Zakat is forbidden. Zaid said: Yes.

This hadeeth is also narrated in other books: Tirmithi, Ahmed, an-nasai, al-hakem and others. Almost all the links are for pages in Arabic. I will translate the important points inshaAllah as I go along.

Mosnad of Ahmed:

Ibn Namir told us that Abd Al-Malik (Ibn Abi-Sulayman) told us according to Attia Al-`ufi who said: I asked Zayd Ibn Arqam and I told him that some people told me a hadeeth according to you about Ali (rah) in the day of Ghadeer Khumm and I want to hear it from you. So Zayd said: You people of Iraq, you have what you have. I told him: Do not worry about me. He said: Yes we were in Al-Juhfa and the prophet(peace be upon him) came to us holding the hands of Ali (rah) and said: O people don't you know that I am <awla> with the believers from their selves. They said: Yes. He said: <man kuntu mawlaah fa Ali mawlaah>.  [Attia] said: Did he say: <Allahumma waali man waalah wa `ad man `adaah> (O Allah: Befriend whosoever befriends him and be the enemy of whosoever is hostile to him). [Zayd] said: I told you as I heard it.

The saying "man kuntu mawlah fa Ali mawlah" is correct and strong.

The addition <allahummu wali man walaah wa `adi man `adaah>, "O Allah: Befriend whosoever befriends him and be the enemy of whosoever is hostile to him" is also correct, albeit weaker.

However, the scholars of hadeeth classify any extra additions as <mawdoo`> and hence we will not talk about. We will only talk about the authentic versions shown above.

This hadeeth is used to prove that Ali is the khalifa. The Shia says that "mawla" means "wali" (leader) and hence this incident means that the prophet is telling the Muslims that Ali is the next khalifa.

This is their point regarding this hadeeth.  The difference, then, is the meaning of "man kuntu mawlah, fa Ali mawlah". The shi`ah says that it means "man kuntu waleeh, fa Ali waleeh". The Sunnah says that it means love and close relation. Muwalat is the opposite of Mu`adat. The proof comes from the first addition: "oh Allah waali man walaah wa `adi man `adaah". (O Allah befriend whosover befriends him and be the enemy of whosoever is hostile to him). So we are talking about muwalah and mu`adat (love and enmity). It is about the love of the people to Ali (rah).

Before we talk about that, let us talk about why the prophet said so.  The Shi`ah claim that the prophet ( peace be upon him ) stopped people in this place in the hot weather and they say that their number was more than 100 thousand and that this was the place where all the people of Hajj were gathered and the main reason is that the prophet ( peace be upon him ) wanted to tell the people "man kuntu mawlah fa Ali mawlah" in addition to their additions.

The reason for this hadeeth is as follows:

Read this hadeeth from Bukhari volume 5, Book 59 Number 637:

Narrated Buraida: The Prophet sent 'Ali to Khalid to bring the Khumus (of the booty) and I hated Ali, and 'Ali had taken a bath (after a sexual act with a slave-girl from the Khumus). I said to Khalid, "Don't you see this (i.e. Ali)?" When we reached the Prophet I mentioned that to him. He said, "O Buraida! Do you hate Ali?" I said, "Yes." He said, "Do you hate him, for he deserves more than that from the Khumlus."

In other narrations (al-tirmithi and ahmed), in addition to the above, the prophet said: "O buraida man kuntu mawlah fa Ali mawlah"

So Ali took one of the slaves named Waseefa as his slave and then after having sexual act with her, he was taking a bath.  Buraida (rah) became angry. He thought why would Ali take one of the slaves for himself. So he mentioned this to the prophet(peace be upon him).

Al-bayhaqi narrates from Abu sa'eed that Ali prevented them from riding the camels of the sadaqa that they acquired. He then made a person to be their leader and went to the prophet(peace be upon him). When Ali came back, he found that their leader gave them permission to ride the camels. When Ali saw that, he became angry and he blamed the leader. (In another narration, it mentions that they were new clothes that Ali prevented them from wearing but they wear them despite his orders). Abu sa'eed said that when we went back to madina, we mentioned to the prophet the harshness that we have seen from Ali. The prophet said: Stop O saad ibn Malik: O By Allah I have known that he is done good for the sake of Allah. (Ibn Katheer says that this hadeeth is <jayed> on the conditions of Al-Nasaie)

Ibn Katheer said that the people in the army started to talk about Ali (rah) because he prevented them from riding the camels and took back the new clothes that they acquired. Because of that, after the prophet ( peace be upon him ) was done with the Hajj and while returning back to Madina, he stopped to explain to the people how some of the qualities of Ali and stress the closeness of Ali to him and the importance of loving Ali.  He did so to remove what was in many of the hearts of some of the people against Ali.

That's why the prophet ( peace be upon him ) delayed talking about this topic until they were close to Madina and he didn't talk about it in Makkah during the Hajj. On the day of Arafa, the prophet ( peace be upon him ) spoke to the people and and he never mentioned this topic at all. After he finished his sermon, he said "Did I convey the message" and the people said "Yes" then he said "O Allah be my witness".

Why did he delay the topic till after Hajj? Because this topic is only of concern to the people of Madina because those who talked about Ali were from Madina as they were the ones who went with Ali to the battle. He talked about it in a place called ghadeer khumm in a place called Ju’fa, which is about 250 km from Makkah. 

A simple look at the map is enough to refute the shi`ah claim that this is the gathering place of the the Hajeej.  People gather for Hajj at Makkah and  leave the Hajj at Makkah. The Hajeej don't leave from a place 250 km from Makkah. After Hajj, the people of Makkah stay at Makkah. The people of Al-Taif go to Al-Taif. The people of Yemen go to Yemen. The people of al-kufa go to al-kufa and so on. All the tribes go back to their homes.  The people that went with the prophet(peace be upon him) were the people of Madinah and those who are using the road of Madina to go to their dwellings.  These are the ones that the prophet( peace be upon him ) talked to when he said: "man ..".

Also the speech was not only about Ali despite the fact that Ali deserves the speech and more (may Allah be pleased with him). But in the speech, the prophet ( peace be upon him ) reminded the people with the Quran and its importance. He also reminded the people of the love of his household (may Allah be pleased with them) and then he mentioned Ali (rah).  So there were more than one topic that the prophet ( peace be upon him ) talked about.

The meaning of ( Mawla )

Ibn Al-Atheer says that the word (mawal) in the Arabic language could only mean:

rabb = Lord

malik = owner

mun`im = benefactor

mu'tiq = liberator

naser = helper

muheb = lover

haleef = ally

aabd = slave (for example: Zaid ibn haretha was the mawla of the prophet ( peace be upon him ))

sihr = brother-in-law

ibn al `am = cousin

The arabs would use the word mawlah to mean all of the above. But what did the prophet ( peace be upon him ) mean by his word?

First the hadeeth has no evidences for the imamah (leadership) because if the prophet ( peace be upon him ) wanted to mean khilafah or imamah, he wouldn't have used a word that can have all these meanings. He would have said something like: Ali is your khalifa (or imam) after me, or when I die, listen and obey to Ali ibn Abi Talib. But the prophet didn't use any of these clear words. He said: "man kuntu ...".

Imam Shafi'i says that this is the walaa of Al-islam because Allah(swt) says in the Quran (Surat Muhammad,verse 11:

That is because Allah is the Protector (mawala) of those who believe, and because the unbelievers shall have no protector for them (47:11)

In the Quran (Surat Al-hadid (The Iron), verse 15), Allah (swt) says:

So today ransom shall not be accepted from you nor from those who disbelieved; your abode is the fire; it is your friend (mawlakum) and evil is the resort.

He called the hell fire "mawla" for the extreme closeness to it by the kuffar.

Note also that the word mawlah is different than the word waly.

The waly comes from walayah which is the leadership. Whereas mawlah comes from wilayah which is love and nusrah (help and aid).

Allah (swt) says in the Quran (Surat At-tahrim ,verse 4):

Then surely Allah it is Who is his Guardian (mawlah), and Jibreel and -the believers that do good, and the angels after that are the aiders.

It means love, nusrah and help.

The prophet ( peace be upon him ) used the word mawla not only to describe Ali but to others as well. The following hadeeth is narrated in Bukhari Volume 4, Book 56, Number 715 & and others.

Narrated Abu Huraira: The Prophet said, "The tribes of Quraish, Al-Ansar, Juhaina, Muzaina, Aslam, Ghifar and Ashja' are my helpers (mawaalii),, and they have no protector (i.e. Master) except Allah and His Apostle."

There are other examples as well but I think that the above is sufficient to make the point.

It is also important to point out that the prophet ( peace be upon him ) did not say "after me" (in any authentic narration). He only said "man kuntu mawlah fa Ali mawlah" without giving any time frame. This means that this fact is timeless. If the prophet ( peace be upon him ) had meant "whoever among you is under my leadership, he is also under the leadership of Ali", which is the meaning that the Shi`a understands, if the prophet had meant it as such, then there would be a big problem. Two leadership for the Muslim ummah at the time of the prophet ( peace be upon him ) does not make a lot of sense.

Of course, the prophet did not mean it that way and also the companions at that time did not understand it that way, otherwise there would be a great fitnah.  However, it is possible to have more than one mawla at the same time - to love, help and aid the prophet and to love help and aid Ali (rah).

I hope that the above is simple and clear.  If, however, you have any particular doubts or confusions, please let me know and inshaAllah I can delve more into it.  The Sunni view is that the hadeeth of ghadir khum has nothing to do with who the leader is after the prophet(peace be upon him). 

It means, however, that Ali is loved by the prophet (peace be upon him) and that we should love him and give nusrah to him. May Allah (swt) join us with Ali and the rest of the companions (may Allah be pleased with them all) on the pool to drink from the honorable hands of the prophet ( peace be upon him ).