In about BC 580, everyone's favorite Babylonian, Nebuchadnezzar, conquered the Southern Kingdom (which was the Kingdom of Judah) and took all of the Jews therein as captives. He shipped them all to Persia, where they served as slaves. The Babylonians were actually fairly kind enslavers in that they allowed the Jews to publicly keep their old religion, rather than forcing a new one on them.

In Babylonia, they encountered Zoroastrianism and wound up incorporating it into their thought. It has had quite a bit of influence on Jewish and thus Christian theology. Before the Jews encountered it, there really was no "adversary" as it were; devils, demons, hell, and so forth were not believed in. In addition, "heavenly" entities like angels were also not thought to be. Basically, they thought that the only big, supernatural entity floating around was Adonai, and that was that.

The Jews were kept in captivity for about 70 years, at which point Cyrus, the king of Babylonia of the time, released them from servitude, allowing them, if they wished, to return to Israel.

The Babylonian Captivity was a major milestone for the Jewish peoples, and is referred to as the exile (not to be confused with their exile at the hands of the Egyptians or Israel's exile at the hands of the Assyrians).