The Qur’an doesn’t call the Christians mushrik (the polytheists). However, it accuses many of them of doing shirk (polytheism). The reason why the Qur’an doesn’t call the Ahl-i Kitab mushrikun is that, despite their shirk, they claimed that what they were doing was not shirk. In other words, they believed that tawhid was the right point of view, and they did not consider their shirk as going against the requirements of tawhid. The mushrikun of Makkah, on the other hand, claimed that what they were doing was shirk and that shirk is the right religion from Allah Almighty. Therefore, they deserved to be called mushrikun.
The condemnation of shirk in the Qur’an is for all types of shirk, whether done by those who admit that what they are doing is shirk or not. In other words, if an individual is doing shirk, whether he is a mushrik, a Jew, a Christian, or a Muslim, the condemnation of shirk in the Qur’an is applicable to all of them.
Shirk primarily means ascribing partners to Allah in His physical existence, His attributes, or His rights. For instance, to believe that God has a son, a daughter, or a wife tanatamounts to doing shirk in His physical existence. To believe that like Allah Almighty there is somebody else who knows everything would be considered as doing shirk in His attributes. Likewise, it is only Allah Almighty’s right that He should be worshipped or prayed to. If a person worships somebody else, then he is doing shirk in His rights.
Courtesy:http://www.islamicissues.info/qa_question.php?qid=93