Operetta (literally, "little opera") is a performance artform similar to opera, but with some of the libretto (words) spoken rather than sung. Instead of moving from one musical part to another, the performers in operetta intersperse the musical segments (e. g. aria, recitative, chorus) with periods of dialogue without any singing or musical accompaniment. Operetta is often considered less "serious" than opera, although this has more to do with the generally comic plots than with the caliber of the music.
Operetta is the precursor of the modern musical comedy. There is a fundamental but subtle distinction between the two forms. An operetta is more of an light opera with acting, whereas a musical is a play with singing. This can best be seen in the performers chosen in the two forms. An operetta's cast will normally be classically trained opera singers; indeed, there is essentially no difference between the scores for an opera and an operetta, except for the operetta's lightness. A musical uses actors who sing, but usually not in an operatic style. Having said that, W.S. Gilbert always preferred to use actors who could sing, for his productions rather than singers who could act, so it isn't an unbreakable distinction.
Operetta grew out of the French opéra comique, the form of opera in use for several centuries by most composers after the decline of tragedie lyrique. Jacques Offenbach is usually credited with having written the first operettas, such as his La Belle Hélène.
The most significant composer of operetta in the German language was the Austrian Johann Strauss, Jr (1825-1899). His first and most famous work in the genre is Die Fledermaus, the most performed operetta in the world. Its libretto was based on a comedy written by Offenbach's librettists. In fact, Strauss may have been convinced to write the operetta by Offenbach. He went on to write 16 others, mostly with great success, although his later librettists were not very talented. His operettas, waltzes, polkas, and marches often have a strongly Viennese style and his great popularity has caused many to think of him as the national composer of Austria. Franz Suppe, a contemporary of Strauss, closely modeled his operettas after Offenbach. The Viennese tradition was carried on by Franz Lehar and Oscar Straus in the 20th century.
Possibly the height of English-language operetta was reached by W. S. Gilbert and Sir Arthur Sullivan, who had a long-running musical collaboration in England during the Victorian era. With Gilbert writing the dialogue and lyrics (similar to the libretto of opera) and Sullivan composing the music, the pair produced operettas which were quite popular at the time, and to some degree since. Works such as The Pirates of Penzance continue to enjoy regular performaces and even some movie adaptations. The pair of composers are popularly referred to as Gilbert and Sullivan.