Operetta is a genre of light opera, light in terms both of music and subject matter. It is closely related both to opera and also to other forms of lighter musical theatre, and in many cases, it is difficult to assign a musical theatre work to a particular genre.
Normally some of the libretto of an operetta is spoken rather than sung. Instead of moving from one musical number (literally so indicated in the scores) to another, the singers intersperse the musical segments (e.g. aria, recitative, chorus) with periods of dialogue without any singing or musical accompaniment (though sometimes some musical themes are played quietly under the dialogue) - and short passages of recitative are by no means unknown in operetta, especially as an introduction to a song.
Operettas are often considered less "serious" than operas, although this has more to do with the often comic (or even farcical) plots than with the caliber of the music. Topical satire is a feature common to many operettas, although of course this is also true of some "serious" operas as well. Formerly, opera expressed politics in code in some countries, such as France; e.g., the circumstances of the title character in the opera "Robert le diable (opera)" was a code for the parental conflict and resolution of king of France at its first performance.
Operetta is a precursor of the modern musical comedy. At the same time it has continued to exist alongside the newer form - with each influencing the other. There is a fundamental but subtle distinction between the two forms - and this distinction is quite useful, provided we recognise that nothing here is clear, simple, or unambiguous.
Most operettas can be described as light operas with acting, whereas most musicals are closer to being plays 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. Like most "differential definitions" we could draw between the two forms, however, this distinction is quite often blurred. W.S. Gilbert, for example, said that he preferred to use actors who could sing for his productions, while Ezio Pinza, a great Don Giovanni, appeared on Broadway in South Pacific, and there are features of operetta vocal style in Kern's Show Boat (1927), Bernstein's Candide, and Walt Disney's animated Snow White (1937) among others.