It's sadly not all that simple, and generations of musicologists have killed each other over this subject
Take many French Operas, for instance, which can contain massive amounts of ballet: They are still operas and not musicals.
Still, the main differences are normally:
Opera vs. Singspiel ("song-play"): Singspiel (e.g. "Die Zauberflote") is a German form of music theatre and has developped from the Opera comique. The main difference to an Opera buffa is that there are no recitatives, but spoken dialogue. Still, it is much closer to opera than to drama or comedy, because the music has a bigger part and is essential to the plot. Carmen is an Opera comique because of the spoken dialogue, although the plot is not comical at all. That's why some people rather call it "Opera lyrique" ...
European Operetta (English, French, German and also the Spanish Zarzuela) is very close to the idea of Opera comique and Singspiel, but you could say that compared to opera, it is straightforward light entertainment and composed as such: The music is developped to be more crowd-pleasing (in its time!) and both lyrics and music are easily understood, even of people who are not that (musically) educated. It is usually composed in the country's native language so everyone can understand it, while opera for a very long time was composed for aristocrats and the so-called educated people, and not necessarily in their first language but rather Italian or French.
Musical in its origin is a strictly American form of music theatre and is quite close to the idea of operettas (and no, originally amplification has nothing to do with it

). Still, there are many types of musicals, so you cannot really throw them all in one pot. They do not necessarily have spoken dialogue in it (think of "Les Mis" as the most popular example), neither do they, in these days, have to consist of very simplistic music (just think of Sondheim!). It is merely a rule of thumb than a strict rule that musicals are light entertainment, but it was the basic idea ...
One could really spend ages in writing down the history of all the different forms of music theatre, but maybe that's enough as a short introduction