SubhanAllah you might unfortunately find that we fit into at least one of these categories without even realising. And without specific and sincere repentance its not something that can be forgiven with our usual istighfar or small acts of good, unlike minor sins.
A Muslim should steer clear of all sins, major and minor ones. A faithful Muslim takes care of every utterance or deed, avoiding anything that makes him subject to the punishment of Allah and His Wrath.
The major sins are those acts which have been forbidden by Allah in the Qur’an and by His Messenger (peace and blessings be upon him) in the Sunnah , and which have been made clear by the actions of the first righteous generation of Muslims, the Companions of the Prophet. Allah Almighty 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." (An-Nisaa’:31)
In this verse, Allah Almighty has guaranteed the Garden of Paradise to those who avoid the major sins. And Allah Almighty also says: "Those who avoid the greatest of sins and indecencies, and forgive when they are angry" (Ash-Shura: 37) and "Those who avoid the greatest sins and indecencies, except for oversights, (will find that) surely your Lord is ample in forgiveness." (An-Najm: 32)
The Messenger of Allah (peace and blessings be upon him) said: "The five (daily) Prayers, Friday to Friday and Ramadan to Ramadan, make atonement for what has happened since the previous one, as long as 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 hadith: "Avoid the seven noxious things: associating anything with Allah, magic, killing one whom Allah has declared inviolate without a just cause, consuming the property of an orphan, devouring riba (interest), turning back when the army advances, and slandering chaste women who are believers but indiscreet." (Al-Bukhari and Muslim)
`Abdullah ibn `Abbas said: "Seventy is closer to their number than seven," and indeed that is correct. The above hadith 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), such as theft, fornication or adultery, and murder; those prohibited acts for which a warning of a severe punishment in the Hereafter is given in the Qur'an or the Sunnah; and also those deeds which are cursed by our Prophet (peace and blessings be upon him). 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 (peace and blessings be upon him) 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 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." (An-Nisaa': 48)
Some of the major sins or kaba’ir are as follows:
1. Associating partners with Allah (Shirk)
2. Committing murder (25:68)
3. Performing Sorcery (2:102)
4. Not performing the Prayers (19:59)
5. Withholding the Zakah (3:180)
6. Breaking the fast of Ramadan or not fasting in that month without a valid excuse.
The Prophet (peace and blessings be upon him) said, “Islam is built upon five pillars: testifying that there is no true god except Allah and that Muhammad is the Messenger of Allah, performing the prayers, paying the Zakah, making the pilgrimage to the house (Hajj), and fasting the month of Ramadan.” (Sahih al-Jami # 2837)
7. Not performing the pilgrimage when one has the ability to do so.
8. Disobeying one’s parents (17:23)
9. Severing ties of kinship. (47:22)
10. Committing adultery or fornication (17:30)
11. Committing sodomy
The Prophet (peace and blessings be upon him) said, “Allah will not look at a person (with pleasure) who commits sodomy with a man or a woman.” (Sahih al-Jami # 7678)
12. Taking or paying interest (2:275)
13. Devouring the wealth of orphans (4:10)
14. Forging statements concerning Allah or forging Hadith (39:60)
15. Fleeing from the battle (8:16)
16. Wrongdoing, deception or oppression on the part of the ruler (42:42)
17. Being arrogant, boastful and vain (16:23)
18. Giving false testimony (25:72)
19. Drinking alcoholic beverages (5:90)
20. Gambling (5:90)
21. Slandering innocent women (24:23)
22. Misappropriating something from the booty (3:161)
23. Stealing (5:38)
24. Committing highway robbery (5:33)
25. Making false oath
The Prophet (peace and blessings be upon him) said, “If someone is ordered to take an oath and he takes a false oath in order to take possession of property of a Muslim, then he will incur Allah's wrath when he meets Him.” (Sahih al-Jami # 6083)
26. Committing oppression
27. Levying illegal taxes
The Prophet (peace and blessings be upon him) said, “Do you know who the bankrupt is? The bankrupt form my nation is the one who appears on the Day of Resurrection having performed the prayers, fasted and paid the Zakah, but had also abused that person, slandered that person, wrongfully taken the wealth of that person and spilled the blood of that person. These people will take from his good deeds. If his good deeds are thereby exhausted, he will be given their sins and then he will be thrown into the hell-fire.” (Sahih al-Jami #87)
28. Consuming forbidden wealth or taking it by any means (2:188)
29. Committing suicide (4:29)
30. Being a perpetual liar (3:61)
31. Ruling by laws other than the laws of Islam (5:44)
32. Engaging in bribery (2:188)
33. Women appearing like men and vice-versa.
The Prophet (peace and blessings be upon him) said, “Allah's curse is upon women who appear like men and upon men who appear like women.” (Sahih al-Jami # 4976)
34. Being a “dayyouth.”
Dayyouth is the one who approves the indecency of his womenfolk and who is void of jealousy or a pimp who facilitates indecency between two people. The Prophet (peace and blessings be upon him) said, “Allah has forbidden the Paradise to three people: the alcoholic, the runaway slave, and the one who is complacent in the face of the evil deeds that his family is performing.” (Sahih al-Jami # 3047)
35. Marrying for the purpose of making a woman allowable for another.
36. Not keeping clean from the remains of urine.
Ibn `Abbas (may Allah be pleased with him) reported that Prophet (peace and blessings be upon him) passed by a grave and said, “These two are being punished and they are not punished for something hard. But it is a great sin. One of them did not keep himself clean form his urine and the other went around spreading tales.” (Sahih al-Jami # 2436)
37. Acting for show (107:4-6)
38. Acquiring knowledge only for worldly gain or concealing knowledge (2:160)
39. Breaching trusts (8:27)
40. Reminding people of one's kindness (2:27)
41. Denying predestination (54:49)
“If Allah were to punish the inhabitants of the heavens and earths, then He would punish and He would not be doing injustice to them. If He were to have mercy on them, His mercy would be greater than their actions. If a person had amount of gold equivalent to Mount Uhud or similar to Mount Uhud and spent it in the Path of Allah, (that spending) would not be accepted form him by Allah until he believes in the preordainment of good and evil. And until he knows that what afflicted him was not going to miss him and what missed him was not going to afflict him. If you were to die with any belief other than that, you would enter the Hellfire.” (Kitab al-Sunnah by Ibn Abu Asi # 245. Albani says that its chain is Sahih)
42. Eavesdropping on other’s private conversation. (49:12)
43. Spreading harmful tales (54:10)
44. Cursing others.
The Prophet (peace and blessings be upon him) said, “Abusing a Muslim is evil and fighting him is disbelief.” (Sahih al-Jami # 3598)
45. Not fulfilling one’s promises.
The Prophet (peace and blessings be upon him) said, “Whoever has a four characteristic is a complete hypocrite. Whoever posses any of these characteristics has the characteristics of hypocrisy until he gives it up; whenever he makes a promise, he breaks it up…” (Al-Bukhari)
46. Believing in what soothsayers & astrologers say.
The Prophet (peace and blessings be upon him) said, “Whoever goes to fortuneteller and asks him about something will not have his prayer accepted for forty nights.” (Sahih al-Jami # 5816)
47. A wife being rebellious to her husband (4:34)
48. Striking one's self, wailing, tearing one's clothing, pulling one's hair & similar deeds as a form of mourning.
The Prophet (peace and blessings be upon him) said, “One who strikes his cheeks or tears his clothing and shouts in the manner of pre-Islamic culture is not one of us.” (Sahih al-Jami # 5713)
49. Being overbearing or taking advantage of the weak, slaves, wives or animals
The Prophet (peace and blessings be upon him) said, “Allah will torture those who torture people in this world.” (Reported by Muslim)
50. Harming neighbors.
The Prophet (peace and blessings be upon him) said, “A person whose neighbor is not safe from his mischief will not enter paradise.” (sahih al-Jami # 7002)
51. Harming and abusing Muslims (33:58)
52. Harming the slaves of Allah.
The Prophet (peace and blessings be upon him) said that Allah said, “Whoever shows enmity to a slave of Mine (Allah's) I shall be at war with him.” (Sahih al-Jami # 1778)
53. Men wearing silk & gold.
The Prophet (peace and blessings be upon him) said, “Gold and silk have been permitted for the females of my nation and forbidden for its males.” (Sahih al-Jami # 209)
The Prophet (saw) also said, “Men who wears silk in this world will have no portion [of heavens] in the hereafter.” (Muslim)
54. Sacrificing animals for other than Allah.
The Prophet (peace and blessings be upon him) said, “The one who sacrifices for other than Allah is cursed by Allah.” (Sahih al-Jami # 4988)
55. Claiming that somebody is one’s father while the claimant knows it is not true.
The Prophet (peace and blessings be upon him) said, “One who claims that someone is his father and knows that it is not true will be forbidden of paradise.” (Sahih al-Jami # 5865)
56. Arguing or quarreling for show & not seeking the truth.
The Prophet (peace and blessings be upon him) said, “Whoever argues in support of something that is wrong and he knows it Allah will be angry with him until he stops.” (Sahih al-Jami # 6073)
57. Not measuring weights properly (83:1-3)
58. Thinking that one is safe from Allah’s planning (7:99)
59. Eating carrion, blood or pork meat (6:145)
60. Continually not performing the Friday prayers and congregational prayers without any valid excuse
The Prophet (peace and blessings be upon him) said, “If people don't stop abandoning the Friday Prayers Allah may seal their hearts and they will become headless.” (Reported by Muslim)
61. Harming others by manipulation one’s bequests (4: 12)
62. Being deceitful or deceptive (35:43)
63. Spying on the Muslims & pointing out their secrets (68:11)
64. Abusing or reviling anyone of the Companions of the Prophet (saw)
The Prophet (peace and blessings be upon him) said, “Do not revile my companions for, by the One in whose hands is my soul, if you were to spend in charity a mountain of gold similar to mount Uhud it would not be equal to a handful or a half a handful (or what they have done).” (Sahih al-Jami # 7187)