In the Bible story, Daniel was one of the Jews who was in captivity in Babylon during the reign of the Neo-Babylonian kings, in the 600s BC. Like the earlier Joseph (and many other characters in stories), Daniel was able to interpret people’s dreams to tell the future. Because God let him interpret dreams, Daniel became a big favorite of King Nebuchadnezzar.
A little later on, Babylon was being ruled by the last Babylonian king Belshazzar. Belshazzar was drinking wine with his lords one night when a disembodied human hand appeared and wrote mysterious letters on the wall of his palace. They said MENE, MENE, TEKEL, U-PHARSIN. Daniel interpreted the letters for Belshazzar, but these letters said that the Babylonian Empire would fall, and that very night (according to the story) the Persians did take over Babylon.
The new Persian king, Darius, made Daniel one of his main helpers. (Though really the first Persian king was Cyrus, not Darius – the story has it wrong.) But other people were jealous and got Darius to make a law that no one could worship any god but Darius himself. Daniel continued to worship his own God, following the Commandments, and Darius was forced to throw Daniel into a pit with lions.
But (says the Bible), because God protected Daniel, the lions refused to eat Daniel, and he came out of the pit safe. Soon Darius (really Cyrus) allowed all the Jews to go back to Jerusalem.
The Persians and the Jews
Esther and Haman
The Jewish holiday of Purim
More about the Persian Empire