An opera is a theatrical piece that tells a story totally through the music. It consists of recitatives which provide the narrative plot line and elaborate chorus singing, along with duets and arias, which are the parts we most remember.
We get the word opera from the Latin and, later, from the Italian, a noun formed from the word operari, "to work." The style evolved in Italy around 1600, and was initially unrealistic, mainly a chance for soloists to show off. In the mid-18th Century the focus shifted to both strong story and exquisite singing. Emotion is an important quality, a trait carried over to "soap operas" which have no singing but plenty of fake tears.