Esther (8:16) relates, "The Jews had light and Simchah and Sason." The English translation of Simchah is "gladness" and the translation of Sason is "joy." I am not aware of an essential difference between these two words in English; it seems to be merely a way of saying the same thing with different words. However, the Vilna Ga'on in his commentary to Esther writes that in Lashon ha'Kodesh there is a significant difference between the two words. "Simchah" refers to the feeling that one has when he is on the way to find what he desires. "Sason" is the feeling that comes afterwards; it is the happiness that one experiences in his heart when he has already achieved that which he sought.