According to the Quran and narrations, there are two types of sins, major sins and minor sins.
According to the Quran and narrations, there are two types of sins:
Major sins such as adultery, treason, drinking wine, killing innocents, usury, interestingly disappointment of Allah's mercy and etc.
In the Quran we read: If you avoid the major sins that you are forbidden, We will absolve you of your misdeeds…[1] (4:31)
The criterion for sins to be Major could be found in the narrations as there is a chapter in Usul al-Kafi titled Chapter of major sins encompassing 24 narrations in this regard. We are able to find the criterion in Imam Khomeini's outstanding fiqhi book Tahrir al-Wasila in which one of the criteria, that is mentioned in the first two narrations of relevant chapter of the Usul al-Kafi as well, is the sins that succeed threat of punishment in the Quran and narrations.
Minor sins such as cursing, looking at non-mahram, entering mosque in the state of Jinabah, and etc.
In the Quran we read: The Book will be set up. Then you will see the guilty apprehensive of what is in it. They will say," Woe to us! What a book is this! It omits nothing, big or small, without enumerating it." They will find present whatever they had done…[2] (18:49) Small sins or minor sins are compared with other sins and it is correct to call them small due to their degrees of punishment or hate of Allah, otherwise all kind of sins are embarrassment and shame for a believer.
As some of the sins are major or minor, Amir al-mu'minin (a.s) states that some of them are the worst as it is related in Nahj a-Balaqa where Imam Ali asserts: The worst sin is that which the committer takes it lightly.[3] Since for a believer the magnitude of sin is not the case but the greatness of before whom he likely commits the sin is the case. The believer does not look at the size of the sin he may commit but he considers the exaltedness of Allah that the small sin takes place.
Notes:
[1] إِن تجَتَنِبُواْ كَبَائرَ مَا تُنهْوْنَ عَنْهُ نُكَفِّرْ عَنكُمْ سَيِّاتِكُمْ وَ نُدْخِلْكُم مُّدْخَلًا كَرِيمًا
[2] وَ وُضِعَ الْكِتابُ فَتَرَى الْمُجْرِمينَ مُشْفِقينَ مِمَّا فيهِ وَ يَقُولُونَ يا وَيْلَتَنا ما لِهذَا الْكِتابِ لا يُغادِرُ صَغيرَةً وَ لا كَبيرَةً إِلاَّ أَحْصاها وَ وَجَدُوا ما عَمِلُوا حاضِراً وَ لا يَظْلِمُ رَبُّكَ أَحَدا
[3] Nahj al-Balaqa. Hadith no 477
وقال عليه السلام : أَشَدُّ الذُّنُوبِ مَا اسْتَخَفَّ بِهِ صَاحِبُه