"Shirk," the word used for idolatry in Arabic means "partnership/association." In the Qur'an, it refers to ascribing associates to Allah, or deeming someone or something, called idols or false-gods, worthy of worship beside or in addition to Allah. Idolatry, though, is not limited to worshipping totems or non-living beings. Because man's responsibility is to serve his Creator, and to strive to earn His good pleasure alone, his pursuit of any other goal is to ascribe worship to something other than Allah. For instance, a person would be guilty of idolatry if he were to seek the pleasure of people instead of Allah's. Likewise, it would again be ascribing associates to Allah if one's purpose in life is to satisfy his whims and desires rather than to earn Allah's good pleasure. Many people ascribe divinity to things like money, status, wealth and so on.
In the Qur'an there is reference to idolaters of Arab society, who set aside a portion of their crops and cattle for their idols, as follows:
As is mentioned in the verse above, idolaters assign a part of their wealth to Allah, and another part to their idols. This is characteristic of the delusion of idolaters.
Loving a being more than Allah or loving him/it as one ought to love Allah is again a form of idolatry. Similarly, someone who fears a being as he should fear Allah, idolizes it, since he assumes that that being possesses a might apart from and independent of Allah.
Muslims, however, firmly believe that everything is created by Allah, that all affairs are regulated by Him, that causes do not have any power to produce a result independently, that every event is predetermined and created by Allah, that Allah is the possessor of the ultimate will and judgment. This is the kind of belief system that Allah reveals to us in the Qur'an. Deviation from these tenets, believing that everything occurs spontaneously, as the result of coincidences, attributing the power to create to some other causes, are all forms of ascribing associates to Allah. Allah does not forgive idolatry:
When Allah's Messenger (may Allah bless him and grant him peace) was asked, "What is the biggest sin in the sight of Allah?", he also said, "To set up rivals unto Allah although He alone created you." (Al-Bukhari)