Christians call the Hebrew Bible the Old Testament.

No. The Christian Bible contains the Hebrew scriptures in what we call the Old Testament. Christ and his teachings and the works of the Apostles are in the New Testament.

Additionally, there are translation differences, additions, and order-changes between the Hebrew Bible or Tanakh and the Christian Old Testament.

The Hebrew Bible is another name for what Christians often call the "Old Testament." Jews do not tend to call their bible the "Old Testament", since this name implies that their bible is "old" or that it was replaced by the New Testament. Jewish people do respect the Christian scriptures but do not regard the New Testament as sacred to Judaism, so a Bible for Jews would only contains the Old Testament. Since they do not have any other Testaments, many Jews call their scriptures the Hebrew Bible or the Tanakh (the Hebrew abbreviation for these scriptures).

As for the King James Bible, it is a popular Christian translation, from the Protestant tradition-- it contains both the Old Testament and the New Testament. You will note that the King James translation of the Old Testament puts the books in a different order from how the Hebrew Bible has them, and some older versions of King James translate certain verses somewhat differently, in an effort to "prove" that Jesus was predicted in the Hebrew scriptures (Jews do not believe he was). But in general, the main difference is the King James Bible contains both the Hebrew Scriptures and the New Testament, while a Hebrew Bible contains the approved Jewish scriptures only, usually translated from the Hebrew by the Jewish Publication Society or another authorized Jewish organization.

The Old Testament (or Hebrew Bible) was written almost entirely in Hebrew. Some parts of the Old Testament were written in Aramaic. The New Testament was written in Greek.

The original text is not called the "Old Testament". It is called "the Hebrew bible.

See related links for the complete Hebrew Bible online.