The major sins are those acts which have been forbidden by Allah in the Quran and by His Messenger (SAW) in the Sunnah (practise of the Prophet), and which have been made clear by the actions of of the first righteous generation of Muslims, the Companions of the Prophet (SAW).
Allah Most High says in His Glorious Book:
If you avoid the major (part) of what you have been forbidden (to do), We will cancel out for you your (other) evil deeds and will admit you (to Paradise) with a noble entry. (al-Nisa 4:31)
Thus by this verse, Allah Most High has guaranteed the Garden of Paradise to those who avoid the major sins. And Allah Most High also says:
Those who avoid the greatest of sins and indecencies, and forgive when they are angry (al-Shra 42:37) Those who avoid the greatest sins and indecencies, except for oversights, (will find that) surely your Lord is ample in forgiveness. (Al-Najm 53:32)
The Messenger of Allah (SAW) said: “The five [daily] prayers, Friday to Friday, and Ramadan to Ramadan make atonement for what has happened since the previous one when major sins have been avoided.” It is therefore very important to determine exactly what the greatest vices, technically called “the major sins” (Kaba’ir), are, in order that Muslims should avoid them.
There is some difference of opinion among scholars in this regard. Some say these major sins are seven, and in support of their position they quote the tradition: “Avoid the seven noxious things”- and after having said this, the propeht (SAW) mentioned them: “associating anything with Allah; magic; killing one whom Allah has declared inviolate without a just case, consuming the property of an orphan, devouring usury, turning back when the army advances, and slandering chaste women who are believers but indiscreet.” (Bukhari and Muslim)
'Abdullah ibn 'Abbas said: “Seventy is closer to their number than seven,” and indeed that is correct. The above tradition does not limit the major sins to those mentioned in it. Rather, it points to the type of sins which fall into the category of “major.” These include those crimes which call for a prescribed punishment (HADD; plural, HUDUD), such as theft, fornication or adultery (ZINA), and murder; those prohibited acts for which a warning of a severe punishment in the Next is given in the Qur’an or the tradition; and also those deeds which are cursed by our Prophet (SAW). These are all major sins.
Of course, there is a gradation among them, since some are more serious than others. We see that the Prophet (SAW) has included SHIRK (associating someone or something with Allah) among them, and from the text of the Qur’an we know that a person who commits SHIRK will not his sin be forgiven and will remain in Hell forever.
Allah Most High says:
Surely, Allah does not forgive associating anything with Him, and He forgives whatever is other than that to whomever He wills. (al-Nisa 4:48 and 116)
- Associating anything with Allah
- Murder
- Practising magic
- Not Praying
- Not paying Zakat
- Not fasting on a Day of Ramadan without excuse
- Not performing Hajj, while being able to do so
- Disrespect to parents
- Abandoning relatives
- Fornication and Adultery
- Homosexuality(sodomy)
- Interest(Riba)
- Wrongfully consuming the property of an orphan
- Lying about Allah and His Messenger
- Running away from the battlefield
- A leader’s deceiving his people and being unjust to them
- Pride and arrogance
- Bearing false witness
- Drinking Khamr (wine)
- Gambling
- Slandering chaste women
- Stealing from the spoils of war
- Stealing
- Highway Robbery
- Taking false oath
- Oppression
- Illegal gain
- Consuming wealth acquired unlawfully
- Committing suicide
- Frequent lying
- Judging unjustly
- Giving and Accepting bribes
- Woman’s imitating man and man’s imitating woman
- Being cuckold
- Marrying a divorced woman in order to make her lawful for the husband
- Not protecting oneself from urine
- Showing-off
- Learning knowledge of the religion for the sake of this world and concealing that knowledge
- Bertrayal of trust
- Recounting favours
- Denying Allah’s Decree
- Listening (to) people’s private conversations
- Carrying tales
- Cursing
- Breaking contracts
- Believing in fortune-tellers and astrologers
- A woman’s bad conduct towards her husband
- Making statues and pictures
- Lamenting, wailing, tearing the clothing, and doing other things of this sort when an affliction befalls
- Treating others unjustly
- Overbearing conduct toward the wife, the servant, the weak, and animals
- Offending one’s neighbour
- Offending and abusing Muslims
- Offending people and having an arrogant attitude toward them
- Trailing one’s garment in pride
- Men’s wearing silk and gold
- A slave’s running away from his master
- Slaughtering an animal which has been dedicated to anyone other than Allah
- To knowingly ascribe one’s paternity to a father other than one’s own
- Arguing and disputing violently
- Witholding excess water
- Giving short weight or measure
- Feeling secure from Allah’s Plan
- Offending Allah’s righteous friends
- Not praying in congregation but praying alone without an excuse
- Persistently missing Friday Prayers without any excuse
- Unsurping the rights of the heir through bequests
- Deceiving and plotting evil
- Spying for the enemy of the Muslims
- Cursing or insulting any of the Companiions of Allah’s Messenger
Source: The Major Sins Al-Kaba’r By Muhammad bin 'Uthman Adh-Dhahabi, rendered into English by Mohammad Moinuddin Siddiqui
http://www.jannah.org/articles/sins.html