Ahasuerus is translated as Xerxes in more modern versions of the Bible such as the NIV. Ahasuerus and Xerxes I are the same person. For more about Xerxes, see here.
There are multiple kings with the name of Xerxes in the Bible. Keeping them straight is a bit of a puzzle. The most prominent Xerxes is Xerxes I who was king during the book of Esther. While some translations use the name Xerxes, other translations – the King James Version being the most notable – use the Persion version of the name, Ahasuerus. To further confuse the matter, this name when translated into Greek is Artaxerxes. But this is not to confuse Xerxes with Artaxerxes who are two different kings.
Suffice it to say, when Xerxes is mentioned it is likely in reference to the King of Persia who married Esther. In some translations his name is Ahasuerus. References to Artaxerxes are most likely to Xerxes’ son who followed him on the throne.
Not only is Xerxes a prominent figure in the book of Esther, but part of the book of Ezra takes place during this same time frame so he also is referenced in that book as well.
Vashti is a queen of the Persian Empire, married to King Xerxes I (called Ahasuerus in the Bible.) Her story is what sets the stage for the book of Esther. During a drunken party she is called out to dance for the king. When she refuses, the king, with some coercion from his guests, decides that it is time to find another queen.
It is speculated that Vashti is not just fighting for women’s rights in her refusal of the king. It is thought that she might have been asked to dance naked for the party guests to show off her beauty. She refused to be humiliated in this way, which is the real force behind going against the king’s wishes.
Vashti ends up being banished from the king’s presence and a search for a new queen was launched. Esther would end up becoming queen at Vashti’s loss. Although one might expect that Vashti was executed for her refusal of the king, this doesn’t happen.