Glitz is a cheap or gaudy flashiness. A sparkly, sequined dress that's really shiny but not very well made has glitz, but it may not be the most tasteful outfit for your grandparents' anniversary party.
The decorating committee for a high school prom might be aiming for sophistication and end up with glitz, with spangles and glitter that's all a bit too much. If someone talks about the "glitz and glamour" of a party, the guests' clothes were probably pretty flashy. In Yiddish, glitz means "glitter," from the German root glitzern, "sparkle" or "glittering." In English, glitzy came first, probably influenced by the word ritzy.