Allah is the Arabic word for God in Abrahamic religions. In English, it generally refers to the God of Islam. It has been used as a term for God by Muslims and Arab Christians alike.
That's a complex question. Islam, Judaism, and Christianity come from the faith of Abraham. They're not the same, as much has been added to Abraham's faith for all three. From a historical point of view, Islam and Christianity and Judaism worship the same God. How they understand God, and how they live that faith, is a very different story.
