Brief Answer:
Sins are generally of two types: (1) major sins and (2) minor sins. It is important to note any sin that involves disobeying the commands of Allah is considered a major sin. The Noble Prophet (ṣ) has said, “Don’t look at whether a sin is minor; rather, look at Whom you are disobeying.” Imām ʿAlī has also said, “The greatest of sins are those which the sinner considers minor.”

Therefore, we see that every sin is a major sin. The terms “major” and “minor” are relative terms used in regard to the effects and outcomes of the sin. Ergo, the division of sins as major or minor does not contradict the aforementioned aḥādīth, as it is used in relation to the effect of the sin and not the sin itself. In spite of this classification, it is important to acknowledge that sin is an act of disobedience against Allah.

In differentiating between major and minor sins Imām Muḥammad al-Bāqir (ʿa) has said, “Every sin for which Allah has promised hellfire as its punishment is a major sin.”

The following āyāt of Qurʾān address these two types of sin:
If you avoid the major sins that you are forbidden, We will absolve you of your misdeeds and admit you to a noble abode.
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.’

The Qurʿān uses different terms to describe different types of sins, based on the nature of that sin. The following are some of these terms:
ذنب، معصية، اثم، سيّئة، جُرم، خطيئة، فسق، فساد، فجور، منکر، فاحشة، and شر
Detailed Answer:
In his book Tahrīr al Wasīlah, Imām Khomeini has mentioned six criteria for major sins:

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Any sin for which the sharīʿah (Islamic law) guarantees hellfire as a punishment
Any sin that the sharīʿah strongly forbids
Any sin that Qurʾān and ḥadīth consider equal to or greater than other major sins
Any sin that one’s intellect or common sense acknowledges as being major
Any sin that Muslims commonly consider a major sin
Any sin that our ḥadīth specifically identify as a major sin.

Amrū ibn ʿAbid, one of the Islamic scholars, approached Imām Jaʿfar aṣ-Ṣādiq (ʿa), gave his salāms and then recited the following āyah of the Qurʾān: “…those who avoid major sins and indecencies…”.

He then became silent and did not complete the verse of Qurʾān. The Imām (ʿa) asked him why he became silent.

Amrū responded, “I would like to know what the major sins are according to the Book of Allah.”

Imām Jaʿfar aṣ-Ṣādiq (ʿa) then listed the major sins that are mentioned in the Noble Qurʾān:
The greatest of the major sins is shirk, attributing partners to Allah: Indeed, whoever ascribes partners to Allah, Allah shall forbid him [entry into] paradise…
Despairing of Allah’s mercy: Indeed no one despairs of Allah’s mercy except the faithless lot.
Feeling safe from the plans of Allah (His punishment and reprieve): No one feels secure from Allah’s plans except the people who are losers (those who shall perish).
Mistreating parents: …and (Allah commanded me) to be good to my mother, and He has not made me self-willed and wretched.
Killing an innocent person: Should anyone kill a believer intentionally, his requital shall be hell, to remain in it [forever]; Allah shall be wrathful at him and curse him and He shall prepare for him a great punishment.
Accusing chaste women of adultery or fornication: Indeed those who accuse chaste and unwary faithful women shall be cursed in this world and the Hereafter, and there shall be a great punishment for them.
Taking and using from the property of orphans: Indeed those who consume the property of orphans wrongfully, only ingest fire into their bellies, and soon they will enter the Blaze.
Fleeing from the battlefield during jihād: Whoever turns his back [to flee] from them that day—unless [he is] diverting to fight or retiring towards another troop—shall certainly earn Allah’s wrath, and his refuge shall be hell, an evil destination.
Practicing usury: Those who exact usury will not stand but like one deranged by the Devil’s touch…..they shall be the inmates of the Fire and they shall remain in it [forever].
Practicing magic and witchcraft: …They would learn that which would harm them and bring them no benefit; though they certainly knew that anyone who buys it has no share in the Hereafter. Surely, evil is that for which they sold their souls, had they known!
Fornicating: …and do not commit fornication. (Whoever does that shall encounter its retribution, the punishment being doubled for him on the Day of Resurrection. In it he will abide in humiliation forever.
Taking a false oath for the purpose of sinning: There shall be no share in the Hereafter for those who sell Allah’s covenant and their oaths for a paltry gain…
Unlawfully taking the spoils of war: …And whoever betrays, [taking unlawfully of the spoils of war], will come with what he took on the Day of Resurrection.
Not paying wājib zakāt (the obligatory poor rate): On the day when these shall be heated in hellfire and therewith branded on their foreheads, their sides, and their backs [and told]: ‘This is what you treasured (hoarded) up for yourselves! So taste what you have treasured (hoarded)!’
Concealing one’s testimony (hiding the truth): … And do not conceal your testimony; anyone who conceals it, his heart will indeed be sinful…
Drinking alcohol: O you who have faith! Indeed wine, gambling, idols, and the divining arrows are abominations of Satan’s doing, so avoid them, so that you may be felicitous.
Deliberately not performing prayer and other wājib acts. The Noble Prophet (ṣ) has said, “Whoever deliberately misses his prayers, has sought estrangement from the promise of Allah and His messenger.
18. And 19. Breaking one’s covenant and severing family ties: But as for those who break Allah’s covenant after having pledged it solemnly, and sever what Allah has commanded to be joined,… is such on whom the curse will lie, and for them will be the ills of the [ultimate] abode.

It is important to note that some of the major sins are greater than others and that their numbers differ in ḥadīth. The difference in number has to do with the fact that the effects of some of the major sins are greater than others.