FORMS OF SHIRK
There are two forms of shirk. One is known as shirk akbar and another as shirk asghar.
Shirk akbar is a 'major' act of shirk that cannot be forgiven by Allah (s.w.t.) and whoever dies while committing it will not be able to enter Jannah.
Shirk asghar are 'minor', more unnoticeable acts of shirk that, if done continuously, might take one 'out of' iman.
SHIRK AKBAR
There are two kinds of shirk akbar -
1. Shirk akbar jali (clear)
This type of shirk akbar is to worship a god or several gods besides Allah (s.w.t.). These 'gods' can be in any kind of form, whether cosmic objects, animals, spirits, jinn or human beings such as priests, kings or rulers.
2. Shirk akbar khafi (subtle)
These types of major shirk include:
* when one prays or supplicates to something other than Allah (s.w.t.)
For example: pious ancestors, dead persons, saints etc. Some say that praying or supplicating to them does not necessarily mean worshipping them. However, Islam stresses that praying or supplicating to something or someone is actually worshipping. (Refer to Tafsir Al Mu'minum:60)
* when one takes a lawgiver or lawmaker other than Allah
This includes all kinds of laws governing human life. The right to make something halal or haram, or to judge in human affairs belongs solely to Allah (s.w.t.). This pertains to both 'religious' and 'secular' in life in Islam. According to Islam, obeying someone while disobeying Allah (s.w.t.), even in the realm of mundane life, is like worshipping it.
SHIRK ASGHAR
There are many forms of shirk asghar or minor shirk. The term 'minor' here does not mean that they are not as grave or serious. Rather it means that these forms of shirk are more inconspicuous and unnoticeable compared to the others.
This form of shirk has been described by the Prophet (s.a.w.s.):
Shirk in the Muslim nation is more inconspicuous than the creeping of the black ant on a black rock in the pitch darkness of the night.
Hence, Muslims should shun and guard themselves very carefully from these kinds of shirk because the most 'minor' shirk may incur a heavier penalty than the most serious sin in Islam. Committing or practicing any act of shirk, whether major or minor, can make one's deeds unacceptable by Allah (s.w.t.).
Common forms of shirk asghar:
* to swear with names other than Allah
This was a common practice of the Arabs during the Prophet's (s.a.w.s.) time. They used to swear with the Ka'aba's name, with a pious ancestor's name, or more commonly, with the names of the idols they worshipped, e.g. Al-Latta, Az-Uzza etc. This is shirk because indirectly it gives some recognition of power or glorification to the being named. Says the Prophet (s.a.w.s.):
Whoever swears with other than Allah, he has committed an act of shirk and kufr.
* to adorn something that is believed to be a form of 'protection from misfortune'
Again, this was a common practice of the early Arabs and is to some extent still prevalent today. Wearing certain bangles, necklaces and amulets was believed to be able to weaken jinn or evil spirits or protect oneself from 'al-ain', misfortune etc. From Imam Ahmad, narrated by Umran bin Hussain:
Once the Prophet (s.a.w.s.) saw a man wearing a yellow bangle and asked him, "What is this?" The man answered, "It is al-wahinah (that which weakens)." The Prophet (s.a.w.s.) said, "Take it off! Verily it will not increase you in anything, except sickness and if you die adorning it, no happiness will come to you forever."
This emphasis and serious attention given by the Prophet (s.a.w.s.) shows the need to shun all forms of shirk and close all doors that might lead to it.
to practice, involve or believe in any form of sihir (sorcery) including charms, incantations, astrology etc.
This is stated in a Hadith:
There are three kinds of people that will not enter paradise: the drinker, the one who acknowledges sorcery and the one who breaks silaturahim (kinship). (Ahmad and Ibn Habban)
Astrology is said to be part of this because of the Hadith:
Whoever partakes in one part of astrology (the art of the stars), verily he has partaken in sorcery. (Abu Dawood)
This, however, does not include using the stars in navigation, traveling, etc.
* to believe in 'fortune-telling'
From a Hadith by Muslim, the Prophet (s.a.w.s.) once said:
Whoever approaches a fortune-teller, inquires him/her and acknowledges him/her, his prayers will not be accepted for forty days.
A Hadith by Abu Dawood relates:
Whoever approaches a fortune-teller and acknowledges him/her that person has committed kufr to revelation from Allah.
* to believe in evil omens
This happens when a person intends to do something, but decides not to do it because of experiencing or 'seeing' certain evil signs or omens. Says the Prophet (s.a.w.s.):
"Whoever cancels one's intention to do something because of attiyarah (an evil omen), that one has committed shirk". The companions asked, "What is its kafarah (penalty for repentance)?" The Prophet (s.a.w.s.) answered, saying, "O Allah, there is no good except from You and there is no misfortune except from You. Indeed, there is no god but You". (Imam Ahmad)
* to have riya
This means to perform acts or to show off in order to gain praise or fame, or for worldly purposes.
* to have no redha
This implies being inwardly dissatisfied with an inevitable condition that has been ordained for one by Allah (s.w.t.); to continuously lament that if one had not done such and such a thing, one would have had a better result.
* to be excessive in one's reverence, respect and 'glorification' of prophets, pious leaders, sheikhs or imams
This has brought to the practice of rituals and ceremonies in glorification of these people and the act of beautifying, building upon, lighting up and even worshipping their graves.
* to use ambiguous words that might involve shirk
This includes phrases like 'In the name of Allah and the name of so and so' (e.g. a leader), or 'if it was not for so and so', or to use a name that is exclusively Allah's (s.w.t.) in naming something or even to curse Allah's creation.
full article here http://www.missionislam.com/knowledge/Shirk.htm