Opera is storytelling through music, libretto and dance. It's commonly thought to be the ancestor to the musical and is difficult to distinguish the differences between modern day opera and musical works. In general, opera is narrative through music and musicals are subject to their dialogue, both spoken and sung.
Considering the first documented usage of the Italian word, opera, was used in the 17th century to convey a "composition in which poetry, dance and music are combined," it colors the way the music was constructed. It makes sense that operas are driven by raw musical performance regardless of language, words or narrative depth. Because their dialogue was constructed through poetry, meaning had to be conveyed through audible emotion in vocal style and musical expression. As operas were popularized through many countries, it was common for librettists to write lyrics and dialogue in foreign languages. The aesthetic of language was as much a part of the performance as the musical score, not only a catalyst for revealing plot.
Operas come in all shapes and sizes. opera seria was one of the first to catch on, which like its name implies, had a very serious and dramatic tone, where opera buffa would follow as the second most attractive form for its more comical and lighthearted execution. France would evolve the genre into the grand opera, which became synonymous with the Paris Opera itself. The acts were extended from two or three to four or five acts. The subject matter was highly dramatic and the musical performance was complex and demanding for both vocalists and instrumentalists. Early French operas introduced ballet as an important storytelling mechanism, often devoting entire scenes to choreographed performances between the orchestra and dancer. As the end of the 18th century approached, the beginning of the Romantic era amplified the grandiose nature of the opera with huge symphonic orchestras, which called for a more forceful vocal technique to sing above the accompaniment.
As composers and writers strive to create new material in an old genre the lines begin to blur where opera ends and musical begins, especially with the popularity of taking something that isn't opera and adding a "dash opera," such as "rock-opera". There are a few notable successes, including Rent, Sweeney Todd and Les Misérables. Although, these aren't classified as opera in the strictest sense or to some people, in any sense, they follow and reference certain operatic tropes, such as through singing, recitative and dramatic circumstance.
Operas and musicals are closely related and in some cases influencers of each other, but in the end each genre has their own sub-genres and they all attract a slightly different audience and serve different purposes. Opera is not any more or less sophisticated or complex than the modern day musical. It is simply different. If musicals do in fact rely on their words, then it wouldn't be a stretch to say it's limited by the language of its audience. When the burden of conveyance is put upon the music, it has the potential to reach a worldwide audience and be appreciated on a more visceral level. This to me is the biggest difference between the two forms. When you choose to speak in a language you are forced to leave someone out, but the more eloquent you are in that language, even people who aren't familiar will be drawn to your voice.
Photo by: Trey Ratcliff