Purim means ‘lots’ (as in drawing lots) and refers to the lottery Haman used to choose the date for the massacre.
The most important feature of Purim is the reading of the Megillah (a scroll containing the story of Esther). As the story is being read, it is a tradition that every time the name of Haman is mentioned everyone boos, hisses, stamps their feet or makes a noise with a rattle. This is to blot out the name of Haman. However, care should be taken that this does not detract from the reading.
Purim is a time of great celebration and lightheartedness.