According to my music appreciation book:
Operetta, or comic opera, combines song, spoken dialogue, and dance with sophisticated musical techniques. Examples are the operettas of the Englishmen W.S. Gilbert and Arthur Sullivan, such as The Mikado,
Musical is a type of theater that fuses script, acting, and spoken dialogue with music, singing, and dancing with scenery, costumes, and spectacle.
The American musical embraces a variety of styles, yet it is a distict type of musical theater, seperate from opera. In contrast to opera, it tends to use simpler harmonies, melodies, and structures; it has more spoken dialogue, and it's songs have a narrower pitch range.
***TAKEN DIRECTLY FROM "MUSIC, AN APPRECIATION"***
"How bout a little black dress?"~hannahshule
"I have a penis, not a vagina." ~munkustrap178