A Gentile is a person who is not Jewish. It is polite to use this word when needed to describe a person who is not Jewish, as in "my friend Robert is a Gentile." Up until about a hundred years ago, it was very hard (and unlikely) for Jews and Gentiles to be close friends. This is because they would be expected to ask each other over for dinner, but Jews follow strict laws about what they can and cannot eat. They can only eat what is kosher.