There are three different kinds of Shirk:
1) The Greater Shirk
2) The Lesser Shirk
3) The Hidden Shirk
THE GREATER SHIRK:
For the one who dies upon it, the Greater Shirk results in the nullification of deeds and eternity in the Hellfire. Allah Almighty says:
But if they had joined in worship others with Allah, all that they used to do would have been of no benefit to them. [Al-An'am 6:88]
And Allaah Almighty says:
It is not for the polytheists to maintain the Masjids of Allah while they witness against their own selves of disbelief. The works of such are in vain and in Fire shall they abide. [At-Tawbah 9:17]
Whoever dies upon the Greater Shirk will not be forgiven, and Paradise will be forbidden for him, as Allah Almighty says:
Verily, Allah forgives not that partners should be set up with him in worship, but He forgives except that (anything else) to whom He pleases. [An-Nisa 4:48]
And Allah Almighty says:
Verily, whoever sets up partners in worship with Allah, then Allah has forbidden Paradise for him, and the Fire will be his abode. And for the wrongdoers there are no helpers. [Al-Ma'idah 5:72]
Here are some of the forms of the Greater Shirk:
i) Supplicating to the dead
ii) Praying or supplicating to idols
iii) Seeking help from idols or the dead
iv) Making an oath by an idol or a dead person
iv) Slaughtering an animal for an idol or a dead person
THE LESSER SHIRK:
It is that which is called Shirk in either the Qur’an or the Sunnah, but which is not from the category of the Greater Shirk. For example doing certain deeds for show-off, swearing by other than Allah, saying, ‘Whatever Allah wills and whatever so-and-so wills,’ and so on. The Prophet (صلى الله عليه وسلم) said:
What I fear most for you is the Lesser Shirk.
When asked what it was, he (صلى الله عليه وسلم) said: Riya’ (showing off).
It was recorded by Imam Ahmad, At-Tabarani, and Al-Bayhaqi from Mahmud bin Lubayd Al-Ansari, may Allah be pleased with him, with a good chain of narration. It was also recorded by at-Tabarani with a good chain, from Mahmud bin Lubayd from Rafi’ bin Khadij from the Prophet (صلى الله عليه وسلم).
The Prophet (صلى الله عليه وسلم) said:
Whoever swears by anything other than Allah has committed Shirk.
It was recorded by Imam Ahmad with an authentic chain, from ‘Umar bin Al-Khattab, may Allah be pleased with him.
Abu Dawud and At-Tirmithi recorded an authentic chain from a narration of Ibn ‘Umar, may Allah be pleased with them, from the Prophet (صلى الله عليه وسلم) that he said:
Whoever swears by other than Allah has indeed disbelieved or committed Shirk
And the Prophet (صلى الله عليه وسلم) said:
Do not say, “What Allah wills and what so and so wills.” Rather say, “What Allah wills, and then what so and so wills.”
It was recorded by Abu Dawud with an authentic chain of narration from Huthaifah bin Al-Yaman, may Allah be pleased with him.
Though this category does not mean that one has left the religion, or that one will spend eternity in the Hellfire, it does mean that one is lacking in the obligatory complete level of Tawhid.
THE HIDDEN SHIRK:
This third category is indicated by the following saying of the Prophet (صلى الله عليه وسلم):
“Shall I not inform you of what I fear more for you than Masih Ad-Dajjal?”
His Companions said: “Yes, O Messenger of Allah.” He (صلى الله عليه وسلم) said:
“Hidden Shirk: a man stands to pray and adorns his prayer because he sees another man watching him.”
It was recorded by Imam Ahmad in his Musnad from Abu Sa’id Al-Khudri, may Allah be pleased with him.
It is also possible to divide Shirk into two categories only, the Greater, and the Lesser.
As for Hidden Shirk, it is general in both, because it sometimes occurs in the Greater Shirk, such as the Shirk of the hypocrites – they hide their false beliefs, showing belief in Islam. This is because they want to show off to others and they are afraid for themselves.
Hidden Shirk also occurs in the Lesser Shirk, for instance in Riya’ (doing deeds for show-off). Examples of when this happens can be found in the two above-mentioned Ahadith, one related by Muhammad bin Lubayd Al-Ansari, may Allah be pleased with him, and the other by Abu Sa’id, may Allah be pleased with him. And success is from Allah.
[Source: Explanation of Important Lessons (for every Muslim) by Shaykh Abdul-Aziz bin Abdullah bin Baz, Darussalam]