Sometimes all of the Jewish teachings are included and referred to as the Torah. The Torah frequently tells of the important link that Jews have to the promised land.
The promised land is located in Israel. Jews that live outside of Israel are thought to be outside of their place. They pray daily for a return to Jerusalem and the promised land. The importance of this land is put well by one author, "Israel has become the center of the universe for Jews of countless generations, and Jerusalem, the epicenter." (Greenberg 450) Jews did not have control over the promised land from 135 AD when the Roman Empire conquered the area until 1948 AD when it was decided by the United Nation that Palestine should be divided into two parts. This allowed Jews to have a portion of land returned to their possession while Arabs retained another portion. Some Orthodox Jews believe it is wrong to establish a Jewish state when the Moshiach has not returned yet. This is not the only disagreement between practicing Jews. Orthodox Jews and the Reform movement have many differences in basic beliefs.
The deciding factor on whether someone is a Jew is one and it is quite complicated. A child born of a Jewish mother is always a Jew even if they do not believe in G-d. A child born of a Jewish father is only a Jew if they go through conversion although some more liberal Jews consider him a Jew if he was raised Jewish. Someone that practices Judaism is not always a Jew. And the children of a non-practicing Jew may not be considered Jewish by the reform movement.
One of the reasons why this becomes so controversial is because it states, in the Torah Deuteronomy 7:3-4, that children will be lost due to mixed marriages. According to a poll of Americans done by the Council of Jewish Federations only 28% of the children from mixed marriages are practicing Judaism. This seems to confirm what is taught in the Torah.
