An operetta is a light musical play: some of the words are spoken, like in an ordinary play, and some are sung. The story is usually funny, sweet, or romantic — not the heavy, tragic feeling of a grand opera, and often shorter too. It was especially loved in the 1800s and early 1900s. Though actually, people stretch the word. Some use operetta, light opera, and even musical as if they were the same thing; musicians would draw sharper lines, but everyday talk is messier. You will see it in theatre programmes and music history. In casual conversation many learners just say musical or 歌劇, which is broader than this word really is.