What do Muslims view as an infidel?

Answer 1

According to Islam, there are three general words used in Arabic to refer to non-believers in an Islamic context. There is Kaafer (often spelled Kafir) which means "denier" or "negator" since they are denying or negating the truth of God. A Kaafer is someone who rejects God in an Islamic context. There is also Mushrik which refers to a polytheist or pagan, i.e. someone who believes in more than one God. Finally, there is Ahl al-Kitaab or Ahl adh-Dhimma which refer to the "People of the Book" or "Protected Peoples", non-Muslim monotheists like Jews and Christians.

The term "infidel" in English is usually used as an analog for Kaafer, not for the other groups, so a person who is not a Muslim is not necessarily an infidel either, he could be a Mushrik or a member of Ahl al-Kitaab. However, the treatment of Mushriks is not terribly different than the treatment of Kaafers and the treatment of the members of Ahl al-Kitaab depends strongly on the particular ruler in place at a given time.

Answer 2

Quran 60:8

That verse explains it all.