Operetta
Operetta is a term which started to be used in the 19th century to describe an opera that was not too serious and often full of fun.  It is often described as “light opera” (“light” meaning “not serious“).  In an opera everything is usually sung.  In an operetta there is a lot of spoken dialogue, and songs and dances are added to this.  Similar traditions already existed in other countries, e.g. in Germany where the Singspiel tradition was popular (e.g. Mozart’s The Magic Flute).