Who is a jew?

One of the special things about Judaism is the label of being a Jew. Can anyone be a jew? Or is it enough for me to convert to Judaism to be a jew?
Jews believe that a Jew is someone who is the child of a Jewish mother. Some groups also accept children of Jewish fathers as Jewish. A Jew traditionally can't lose the status of being a Jew by adopting another faith, but they do lose the religious element of their Jewish identity. It is also possible for someone who isn't born a Jew can convert to Judaism, but it is very difficult to do so.