What is shirk?
Shirk is to associate someone other than Allaah in those aspects which are unique to Allaah and His exclusive right. Shirk is to worship created beings like Allaah is worshipped, to venerate created beings like Allaah must be venerated, and to assign a portion of His divinity to someone else.
Severity of Shirk
There is no issue upon which Islaam is so strict as the one of Tawheed (monotheism). Therefore, shirk is considered the greatest violation with which the Lord of the heavens and the earth is defied.