How is singing operette different from singing opera?

Opera-singers are primarily singers and then actors, while the singer in an operetta is primarily an actor who is also singing, he doesn’t need to have a perfect trained voice like an opera singer has.

Not every singer of an operetta will be able to sing and play in an opera, but every opera singer should be able to sing an operetta, with his trained or natural voice.

The operetta singer uses his trained voice like his natural voice like an instrument of an actor. The opera singer - in the opera - mostly uses his trained voice like a music instrument.

The operetta is standing nearer to musicals.

How is singing operette different from singing opera?

Operetta (note that the English word comes from Italian, not French) and, later, musical theater, are less demanding. The songs may be musically simpler, but more importantly they tend to require less vocal stamina. The opera orchestra tends to be bigger. Operatic tessitura tends to be higher. Operas are sung completely, or nearly so, while operetta typically has spoken dialogue.

Accordingly, fewer people develop the vocal technique and stamina required for a successful operatic career than those required for a successful career in operetta or musical theater. (Furthermore, through the 20th century, musical theater has evolved along with popular styles, moving towards singing techniques appropriate for electronic amplification.)

There have been examples of people performing successfully in both spheres, of course, and none of these differences are absolute.