Some have requested definitions:
Costumes - People wear costumes for Purim. Esther is popular with kids.
Drunk - see Religious obligation
Grogger - noise maker. Twirl it to drown out Haman's name when it occurs in the reading of the megillah.
Haman - Villain in the book of Esther who tries to have the Jews killed. When the megillah of Esther is read those at the reading use groggers and make noises to blot out his name whenever it occurs.
Hamentaschen - Haman cookies. Start with a round piece of cookie dough, add some filling (Lechvar - prune and mohn - poppy seed are traditional. Apricot, cherry, chocolate chips, almond paste, peanut butter, etc. can also be used.) and fold up edges to make a triangular cookie. Haman is pictured as wearing a three cornered hat.
Megillah - a little scroll. Esther, Song of Songs, Ruth, Lamentations, and Ecclesiastes are spoken of as the five megillot.
Purim - Spring festival which celebrates the events described in Esther.
Purimspiel - A Purim play, performed before, during, or after the reading of the megillah: sometimes fairly straight, but often broadly satiric.
Religious obligation - An old tradition suggests drinking on Purim until you can't remember what you're celebrating.
Shalach manos - Gift of various foods and drink.
Shul - the synagogue.