An opera is primarily sung, whereas in a musical, the songs are interspersed with passages of dialogue. In both instances, it is drama and words that drive the action.
Both operas and musicals use librettos, i.e. texts, as their basis, but in the case of opera, the singing tends to be continuous, whereas in musicals, much of the plot is unravelled through the spoken scenes around the individual songs. There can often be bigger dancing numbers in musicals.
In opera, the singing is split between arias, recitatives and bigger chorus numbers. An aria is a solo vocal piece, in which the character will express personal emotions. They tend to be the more famous, memorable tunes in an opera or oratorio. A recitative, on the other hand, is often sung in speech rhythms and is used as more of a storytelling technique.
In opera, music is at the forefront, whereas in musical theatre, the words are key to the plot development. This is why audiences have been watching and listening to operas in foreign languages for so many years: an understanding of the specific language is often seen as secondary to the music itself.
Some musicals are closer in style to operas than others: Stephen Sondheim’s Sweeney Todd is one such musical that is often categorised as an opera, because of its focus on the libretto and its limited speech.
Voice types in opera often differ to those in musical theatre, using much more vibrato. This is because of the long history of the form, which was performed before microphones were created, so opera singers had to project over the orchestra without amplification. Musical theatre is a newer art form, so its singers are often given microphones to help them be heard over a loud band or orchestra.