What is Shirk?..Concise answer
Literally, shirk means to allocate; technically, in Quranic terminology, shirk – in contrast to hanif – signifies the process of allocating someone/thing as the Almighty Allah’s partner or equal. Hanif means being inclined towards righteousness and moderation; hence, the term has been coined onto those who have disassociated themselves from shirk, abiding by pure tawhid.
Shirk may be broadly classified into (a) shirk relating to beliefs (‘doctrinal’ shirk) and (b) shirk relating to acts of worship/obedience (‘submissive’ shirk). Doctrinal shirk may be divided into:
(i) ‘divine’ shirk: believing in an existent, other than Allah, who independently beholds all divine attributes
(ii) ‘creationist’ shirk: believing that the universe has more than one origin and more than one independent creator
(iii) ‘sovereign’ shirk: believing that the universe is managed by more than one independent Lord
Submissive shirk is categorized into overt and covert divisions. All divisions of shirk are deemed aberrant, before Allah, and are regarded as a great injustice to the Almighty.
Today, one observes some sects abusing the concept of shirk in order to fault the beliefs of others; whenever they fall short of demonstrating proof, they accuse others of shirk. Such behaviour is entirely un-Islamic, immoral and improper. Nevertheless, Islamic scholars have, indeed, replied to all their deviating thoughts and accusing outbursts.
The word itself, Shirk, means to give a portion or share, or the involvement of two partners. The Quranic term has been used in face of "Hanif" (orthodox Muslim), and what is meant by shirk is - to associate a partner or an equal for God (Exalted be his praise). The term Hanif means to have inclination towards truth and righteousness from misguidance. And since the followers of Towhid wholly oppose the idea of shirk and instead are inclined towards this fundamental principal [of Towhid], they are called Hanif. And this is why one of the meanings of Hanif is "straight" and "clear"
In the generous Quran, Allah The Most High has said to the Prophet: Say, 'Indeed my Lord has guided me to a straight path, the upright religion, the creed of Abraham, a Hanif, and he was not one of the polytheists'. God has also stated: "Dedicate yourself to the religion as a Hanif, and never be one of the polytheist."
Hence from a Quranic standpoint; in religion shirk is the opposite of Hanif, and in order to recognize shirk one must recognize Hanif, by identifying an entity by its contrary, meaning that one can distinguish something by distinguishing its opposite. In brief it can be said that shirk is the opposite of Towhid, and just like Towhid has different types, so does shirk. In a general categorization, shirk can be divided into two types: shirk in 'Amal' or actions (such as; obedience and worship), and shirk in 'Aqideh' (belief). Shirk in belief is divided into three parts:
Shirk in Deity: Believing in an existence other than God, which independently has all of the divine qualities. Such a belief will be the cause of kufr
, which is why Allah states in the Generous Quran: They are certainly faithless who say, 'Allah is the Messiah, son of Mary.' Say, 'Who can avail anything against Allah should He wish to destroy the Messiah, son of Mary and this mother and everyone upon the earth?' To Allah belong the kingdom of the heavens and the earth and whatever is between them. He creates whatever He wishes and Allah has power over all things.
Shirk in Creatorship: Believing in two independent origins and creators for the world in such a way that they control creation and modifications made to creation. Like the Mazdakians (Zoroasters) who believe in two originators, one of good "Yazdan" (God), and one of evil "Ahruman" (the devil).
Shirk in Lordship: To believe that there are multiple lords and that God is the Lord of lords. Meaning that managing the affairs of the world has been delegated to each of the independent and individual lords. Just like the polytheist of Hazrat Ibrahim's (Peace be upon him) time who practiced this type of shirk. A group of them recognized the stars as the governor of the world and a group of others recognized the moon and another group the sun.
Shirk in stage of "Amal" (action):
This is known as "shirk in worship and obedience", Meaning that a person's humility and humbleness arises from belief in the deity, creatorship or Lordship of those whom he respects and has humility and humbleness for.
These are the criterion and standards of shirk as it has been mentioned in the Holy Quran. However there are some people who make up standards and criteria for shirk, and then commence to accuse other Muslims of practicing polytheism. These views that some have taken it upon themselves to gather have no type of credibility, as their criteria contradicts the ayats of the Holy Quran, the Seereh (Way and life) of the dear Prophet of Islam (Peace be upon him), and his successors (The Twelve Saint Imams).
Here we will mention some of the 'made-up' standards for shirk:
believing in a super naturalistic sovereignty for other than Allah; they say, "If someone were to implore the Prophet or any of the of Allah's Saints for assistance, and if he believes that they can hear this supplication or have knowledge of his situation or are able to grant what he needs, then this is an eminent type of shirk."
Requesting a need from the deceased. They have said that; "Requesting things from the deceased, seeking their help, and favoring the deceased are all types of shirk, and these are the principals and basis for polytheism."
Dua and imploration are types of worship; they say that, "Worship is only for Allah and Dua is a type of worship; therefore asking for something from other than Allah is also shirk."
Visitation of tombs or graves is shirk.
Seeking blessing and sacring through the effects of the prophets and righteous persons is shirk.
Celebrating the birthday of the prophet is shirk.
Building a dome above a grave is shirk.
These made-up standards and beliefs can be broken into two parts:
One part of these actions and standards are called polytheist actions, because they recognize them as being shirk in belief.
In rebuttal to this part of their beliefs, it can be said that if believing in super naturalistic sovereignty, believing in intercession, and believing in granting of needs, etc. is such that all of these matters are related to Allah, and all that anyone has is from Allah, The Most High, then this is not shirk, because no type of sovereignty other than that of Allah's has been taken into consideration. In the definition of shirk in Deity, shirk in Lordship and shirk in Creation, we said that these various types of shirk take place when one believes that someone other than Allah independently has the [divine] attributes of glory and perfection or can be an independent creator or can independently manage the affairs of this world. However, if one's power or authority was dependent upon Allah, then that would no longer be shirk. We and all other Muslims that request needs from the Noble Prophet and his successors and or that believe that they have mystical strengths, also believe that this position and status has been bestowed upon them from The One Lord, The Most High. Considering this description can we still call this shirk?
As for the other section of actions that they regard as shirk; this is because they recognize these actions as being a kind of worship; i.e. Celebrating the birth of the Prophet (P.B.U.H.), Building a dome or cupola over a grave or kissing a shrine.
Worship has certain mannerisms, and those mannerisms are specific to Allah. Worship denoted by humbleness and humility which comes from one's belief in Deity, Creatorship or Lordship. However, if humility and humbles did not arise from such beliefs then by no means will that be 'worship'. This is why in Sura Yusuf, Allah does not recognize the prostration that Hazrat Yusuf's brothers made before him as shirk, for the reason that they never had a belief of Deity, Creatorship or Lordship towards Hazrat Yusuf (Peace be upon him).
Fortunately, scholars of Islam and other conscious scholars have given rebuttals to all of these cases and fabricated principles.
For more information you can make reference to "Quranic Discussions in Towhid and Shirk", written by Dr. Allama Jafar Subhani.
 Majma'ul Bahrain – Vol.5, Pg.274; Al-Ain – Vol.5, Pg.293
The word 'Hanif' comes form the word 'Hanafa' meaning – inclination towards truth and righteousness form misguidance. However 'Hanafa' actually means –inclination from truth to falsehood. Since the followers of pure Towhid, have turned away from that misguidance and have embraced this basic principal (Towhid) then they are called 'Hanif'. This is also why one of the meanings of Hanif is straight and clear.
 Sura An'aam –Verse 161
 Sura Yunus – Verse 105
 It is necessary to mention that all of these different types of shirk cause kufr. What is meant by kufr here is more general that what legal authorities and theologians recognize as kufr.
 Sura Ma'edeh – Verse 17
Collection of the Fatwas of Ibn Baz – Vol.2, Pg.552
 Fathul Majid – Pg.68
 Arrad Ala Rafezah – According to the Shia narration – Ali Asghar Rezwani – Pg.135 - 143