This is a great book of the Bible for showing just how perfectly God takes care of His people. God's name is never mentioned in the 10 chapters of Esther but His influence is always there, providing a way for the Jewish people to be saved.
The book begins with setting up the situation in the kingdom of Persia. King Ahasuerus (also called Xerxes, I believe) rules over 127 provinces, so his empire is pretty glorious. He throws a giant party for a bunch of men in the palace, and his wife, Queen Vashti, throws a party for the women. On the seventh day of partying, the king calls his wife to come see him, essentially so that he can show her off and brag to his friends about how pretty she is. Vashti says she's not coming, and the king has her thrown out so he can start the search for a new queen to replace her. From the verses, it's not clear whether Vashti was going to be asked to do anything degrading, but the point of this chapter is not about whether she was justified in her choice to disregard the king's summons, it's the fact that her absence makes room for Esther to take her place and do something great.
Esther is among the many pretty girls who are rounded up and taken to the palace during the great search for the new queen. Esther is a Jewish orphan, raised by her cousin Mordecai who works as a gatekeeper for the king. Mordecai advises Esther to keep her ethnicity a secret, and she's the kind of woman who listens to wise counsel. After 12 months of purification (I have no idea what this is...apparently, spending time with the king requires months of rituals involving myrrh), Esther goes to see the king and he likes her so much, he makes her queen. Mordecai overhears a plot to kill the king and he tells Esther, who is now in a position to tell the king. So the king actually owes his cool new wife and his freshly-saved life to Mordecai, but for some reason, he forgets to reward Mordecai for warning him about the plot.
In the meanwhile, the king gets an evil adviser named Haman. Haman hates Mordecai because he won't bow down and reverence him, but he's not content to just kill Mordecai--he concocts a plan to have the king agree to kill all the Jews in all the provinces of Persia. The king seems to trust Haman enough to not check about the details--Haman says "I know of some folks that need killing" and the king says "Okay, go kill them". The king doesn't even know that he has agreed to kill the Jews, but everyone in his royal city of Shushan is puzzled by this decreed massacre.
And here's where the bravery comes in. Mordecai tells Esther that she must intervene for her people before the king. Esther points out that anybody, herself included, who approaches the king without being called will be executed unless the king feels merciful. Trouble is, the king hasn't called for Esther in a full month. Mordecai replies, "who knoweth whether though art come to the kingdom for such a time as this?" He means that perhaps the entire reason that Esther was living in Persia and ended up marrying the king of Persia by the weirdest of happenstances, was so that she could be in a place where she could save the Jews from harm.
Esther makes a series of small requests of the king and invites him to a couple of private banquets before making her big petition: she'd like for the king to spare her life and the lives of her people. Now the tables have turned on Haman. The king has him hanged on the same gallows that Haman had prepared for Mordecai. Also, though the king can't quite undo the previous proclamation, he sends out a new one that says that when the Jews are attacked, they are to defend themselves and take their enemies' stuff as spoil. The day comes and the Jews throughout the king's provinces defend themselves and kill their enemies, but they don't take any spoil (again, I'm not sure why--maybe it was just the honorable thing to do). The Jewish people began celebrating the holiday of Purim on the anniversary of the days that were "turned unto them from sorrow to joy, and mourning into a good day: that they should make them days of feasting and joy, and of sending portions one to another, and gifts to the poor" (Esther 9: 22). Purim celebrates the time that an early Holocaust was averted, and reading the book of Esther can remind us all of how God can use an intelligent, brave woman to bring about great things.