It began as an ethnic group descended from Abraham & Sarah, Isaac and Jacob (who was renamed Israel by the angel of God he wrestled with). Today it's a religious faith, an ethically based way of life. A person by their law is considered to be Jewish (even if they don't practice the faith, as long as they have not converted to something else) if his/her mother is Jewish. A person can convert to Judaism & it is treated as a form of adoption into the group, possibly taking on the names of Abraham & Sarah and the male is circumsized if he hasn't had it done previously. Incidently, for many years, they did not encourage conversions since their way of life they consider difficult - and during times of anti-Semitism, it was actually illegal to perform conversions.