What is the definition of shirk?
Is it a sin? If yes, what kind of sin is it? What are the types of shirk?
Can you give me some examples?
What is the definition of shirk?
Definition of Shirk
The word "shirk (شرك)" means "to be partners", "to be partner in something", "to share something", "to be associated with".
Shirk is holding anything partner to Allah, in both/either body or properties.
Shirk is believing that something only Allah can do, can be done by someone/something else.
If shirk is done by explicitely worshiping a being other than Allah, then that kind of shirk is called "Greater Shirk" or "Open Shirk".
Or, if it is done by believing someone/something can do things which only Allah is able to do, but by mouth he says that there is no ilah (god) other than Allah, then this kind of shirk is called "Concealed Shirk" or "Lesser Shirk".
Examples of Shirk
يَا أَيُّهَا النَّاسُ اذْكُرُوا نِعْمَتَ اللَّهِ عَلَيْكُمْ هَلْ مِنْ خَالِقٍ غَيْرُ اللَّهِ يَرْزُقُكُم مِّنَ السَّمَاء وَالْأَرْضِ لَا إِلَهَ إِلَّا هُوَ فَأَنَّى تُؤْفَكُونَ
O mankind, remember the favor of Allah upon you. Is there any creator other than Allah who provides for you from the heaven and earth? There is no deity except Him, so how are you deluded?
Fatir 3 (35/3)
Allah clearly states that He is the one who gives us blessings (رزق; all kinds of requirements to live and stay alive; like food, shelter, conditions, breath, etc). If someone believes that someone/something else is supplying him these blessings; for example, if he believes that money, his boss, his friends, her husband or his family is supplying him/her the blessings then this belief is called "shirk".
قُلْ أَرَأَيْتُم مَّا تَدْعُونَ مِن دُونِ اللَّهِ أَرُونِي مَاذَا خَلَقُوا مِنَ الْأَرْضِ أَمْ لَهُمْ شِرْكٌ فِي السَّمَاوَاتِ اِئْتُونِي بِكِتَابٍ مِّن قَبْلِ هَذَا أَوْ أَثَارَةٍ مِّنْ عِلْمٍ إِن كُنتُمْ صَادِقِينَ
Say, [O Muhammad], "Have you considered that which you invoke besides Allah ? Show me what they have created of the earth; or did they have partnership in [creation of] the heavens? Bring me a scripture [revealed] before this or a [remaining] trace of knowledge, if you should be truthful."
وَمَنْ أَضَلُّ مِمَّن يَدْعُو مِن دُونِ اللَّهِ مَن لَّا يَسْتَجِيبُ لَهُ إِلَى يَومِ الْقِيَامَةِ وَهُمْ عَن دُعَائِهِمْ غَافِلُونَ
And who is more astray than he who invokes besides Allah those who will not respond to him until the Day of Resurrection, and they, of their invocation, are unaware.
وَإِذَا حُشِرَ النَّاسُ كَانُوا لَهُمْ أَعْدَاء وَكَانُوا بِعِبَادَتِهِمْ كَافِرِينَ And when the people are gathered [that Day], they [who were invoked] will be enemies to them, and they will be deniers of their worship.
Ahkaf 4-6 (46 / 4-6)
Some people believe that when they invoke their elders or sages (dead or alive), they hear them, and they send help to them. This kind of "hearing and sending help" is only a property of Allah. Having a belief that someone or something is able to hear them in such a way and send them help is nothing but holding them partner to Allah. This is a kind of shirk.
وَيَجْعَلُونَ لِمَا لاَ يَعْلَمُونَ نَصِيبًا مِّمَّا رَزَقْنَاهُمْ تَاللّهِ لَتُسْأَلُنَّ عَمَّا كُنتُمْ تَفْتَرُونَ
And they assign to what they do not know a portion of that which We have provided them. By Allah , you will surely be questioned about what you used to invent.
وَيَجْعَلُونَ لِلّهِ الْبَنَاتِ سُبْحَانَهُ وَلَهُم مَّا يَشْتَهُونَ
And they attribute to Allah daughters - exalted is He - and for them is what they desire.
Nahl 56-57 (16 / 56-57)
إِنَّ الَّذِينَ لَا يُؤْمِنُونَ بِالْآخِرَةِ لَيُسَمُّونَ الْمَلَائِكَةَ تَسْمِيَةَ الْأُنثَى
Indeed, those who do not believe in the Hereafter name the angels female names.
وَمَا لَهُم بِهِ مِنْ عِلْمٍ إِن يَتَّبِعُونَ إِلَّا الظَّنَّ وَإِنَّ الظَّنَّ لَا يُغْنِي مِنَ الْحَقِّ شَيْئًا
And they have thereof no knowledge. They follow not except assumption, and indeed, assumption avails not against the truth at all.
Necm 27-28 (53 / 27-28)
Before Nabi Muhammad (pbuh) became a prophet, people believed that females are meant to be daughters of Allah, and they were meant to be angels. They believed that Man mustn't have daughters, Man must only have sons. People were accepting their first daughters, and burying the second (and so on) alive. Having daughter was a shameful act for them because of this.
In Quran, Allah says that this belief is wrong. Allah does never have daughters, and angels are not female.
This (assigning daughters to Allah) is another kind of shirk.
Shirk in Islam
Allah is the only ilah (god). There is no ilah other than Him. This belief is called "Tawheed". Tawheed is the main basis of Islam, and is the most important subject.
Those who violate Tawheed and have shirk in their belief are called "mushrik".
A mushrik may accept that Allah is the greatest of all, and may pray to Allah; but that's not enough to be a Mumin (believer in the safe line). In order to be a Mumin, one must accept Allah as vahid (only one, with no partners, all are done by Him).
Nabi Muhammed was born in a mushrik society. He made a lot of wars and conflicts against them even though they were accepting Allah as the greatest ilah (god).
وَلَئِن سَأَلْتَهُم مَّنْ خَلَقَ السَّمَاوَاتِ وَالْأَرْضَ لَيَقُولُنَّ اللَّهُ قُلْ أَفَرَأَيْتُم مَّا تَدْعُونَ مِن دُونِ اللَّهِ إِنْ أَرَادَنِيَ اللَّهُ بِضُرٍّ هَلْ هُنَّ كَاشِفَاتُ ضُرِّهِ أَوْ أَرَادَنِي بِرَحْمَةٍ هَلْ هُنَّ مُمْسِكَاتُ رَحْمَتِهِ قُلْ حَسْبِيَ اللَّهُ عَلَيْهِ يَتَوَكَّلُ الْمُتَوَكِّلُونَ
And if you asked them, "Who created the heavens and the earth?" they would surely say, " Allah ." Say, "Then have you considered what you invoke besides Allah ? If Allah intended me harm, are they removers of His harm; or if He intended me mercy, are they withholders of His mercy?" Say, "Sufficient for me is Allah ; upon Him [alone] rely the [wise] reliers."
Zumer 38 (39/38)
And, it is good to mention this here, the main problem between Islam and Christianity is that, the Christians believe that Allah has a son. This is shirk in Islam, and is not acceptable.
وَقَالُوا اتَّخَذَ الرَّحْمَنُ وَلَدًا سُبْحَانَهُ بَلْ عِبَادٌ مُّكْرَمُونَ
And they say, "The Most Merciful has taken a son.". Exalted is He! Rather, they are [but] honored servants.
Anbiya 26 (21/26)
Shirk is the greatest sin. Allah says that He will not forgive the sin of shirk.
إِنَّ اللّهَ لاَ يَغْفِرُ أَن يُشْرَكَ بِهِ وَيَغْفِرُ مَا دُونَ ذَلِكَ لِمَن يَشَاء وَمَن يُشْرِكْ بِاللّهِ فَقَدْ ضَلَّ ضَلاَلاً بَعِيدًا
Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills. And he who associates others with Allah has certainly gone far astray.
Nisa 116 (4/116)