As a musician, I am usually over critical of music. That being said. I am also over critical with genre identification. Especially when it comes to musicals and operas. So what constitutes and opera and what constitutes a musical, or even an operetta? I want to say that I mainly sing classical music, but I still enjoy so many other types of music than just what we consider classical.

Opera:

• a genre of classical music.
• a building for the performance of opera.
• a dramatic work in one or more acts, set to music for singers and instrumentalists.

A Dramatic work? Couldn't that be identified as a musical too? Also, singers and instrumentalists also participate in musicals? So, what's the difference? According to an opera singer, it can be the complicity and lack of speaking, but not all operas have a recitative. Some do have speaking - more modern operas, and Operettas. There is also a certain staging and story line that you must have in an opera and another in a musical. The Phantom of the Opera is NOT an Opera. 

Musicals:

• a play or movie in which singing and dancing play an essential part. Musicals developed from light opera in the early 20th century.

Developed from an opera... This is true. Now, musicals are not bad but the classification of them can be different. To most, though, a musical has beautiful music, but also the speaking parts are prominent. This seems to have more of a prominence - the differentiation of the speaking and singing. A cool thing about musicals is that they may be derived from operas, or the same story operas came from like Rent coming from La Boheme. Both are awesome, by the way.

Operetta:

• a short opera, usually on a light or humorous theme and typically having spoken dialogue. Notable composers of operettas include Offenbach, Johann Strauss, Lehár, and Gilbert and Sullivan.

This one is difficult to explain because operettas are performed in both the musical and the opera worlds. A notable operetta is The Pirates of Penzance. To me, an operetta is a cross between a musical and an opera. It is shorter than a typical opera but can contain more music than a typical musical. The keyword is can.

Still, don't know what the difference is? That is okay. Most people won't, but you should at least understand that these are different genres just like pop, country, hip-hop, etc. and if we get upset because you call Peter Grimes a musical, then we have a right to be upset.