Nanees,
That's not what i was getting act. Christians are disbelievers.
Alpha,
maybe my arabic is off. Mushrikiin, are polytheists/idolaters? or anyone who commits shirk. If the latter, what is the difference between a mushrik and a kaffir?
Those are one and the same. Shirk is to associate partners with Allah, to claim He has children, parents, spouse, etc.
Kufr, is the rejection or hiding of the truth. Similar to the "kaafir" in reference to a farmer, who hides seeds for fruit under the soil. Kufaar (kaafir, pl) reject what Islam has come with. They may not always be mushrikeen, since they may not worship anything at all, unless it's argued they worship themselves/life/science/money &c.
You have to understand that the Mushrikeen of Makkah also claimed to believe in Allah, in one god. They claimed that their idols were only a means of achieving nearness to Allah, regardless if their idols were complete fabrications or references to ancestors. Some even worshiped jinn, trees, rocks, etc. I make no* distinction between them and the those from the people of Musa who worshiped the golden calf. Or those who claimed Isa was the son of Allah. But that's just me.