It is important for a Muslim to have some understanding of the major (kaba'ir) and minor (sagha'ir) sins, what the major sins are, the difference between these two categories of sin and how ultimately to avoid them. The quality and level of our life Tawba – repentance really depends on this knowledge.
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 says ‘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 pardon/ forgiveness for what has happened since the previous one when major sins have been avoided."
So If we perform our daily salah (and fasting Month of Ramadan) one of the consequences is that our minor sins are forgiven on condition that we also refrain from the major sins.
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 prophet (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.
Through the ages the scholars have collated and made mention of the major sins in various texts - one such text is the famous work of Imam Ad-Dhahabi which has been summarised in short with reference to relevant evidences from the Quran and Sunnah.