In a discussion around different kinds of musical theatre, I think the first step is to define the terrain of musical theatre. Musical theatre does not include all theatre that uses music. Musical theatre excludes – for example – opera, ballet, dance shows and plays with music.
The next step is to consider what we could broadly term narrative vs non-narrative musical theatre: this separates shows shows that deal with narrative (but not necessarily plot in the conventional sense) from compilation revues, vaudeville, burlesque and so on.
Once we’re in the terrain of narrative musical theatre, it is incredibly important to discern between categories that relate to form (operetta, musical comedy, musical play, concept musical, rock opera) and categories that relate to style (such as mega-musical, minimalist) or content (like jukebox musical or bio-musical) or narrative style (lyric, dramatic or epic).
The key to telling the difference is that the categories a form can be produced using any of the styles or bodies of content.
For instance, Jesus Christ Superstar – a rock opera – can be produced either as a megamusical or in a minimalist fashion, as could any operetta, musical comedy, musical play or concept musical. Consequently, a category defining the style of a production is secondary to one the defines its form.
In terms of content, a jukebox musical can take any the form of an operetta (look at all those shows that use music by classical composers to tell a story) or musical comedy (Mamma Mia!). The principle applies to musical plays, concept musicals and rock operas too. (To link with the example above, a musical about Jesus could, in theory, also be constructed in a jukebox fashion.) Similarly, a bio-musical might be constructed as a musical comedy (Funny Girl), a musical play (Gypsy) or a rock opera (the above-mentioned Jesus Christ Superstar). As such, categories related to content are also secondary to those related to form.
In terms of narrative style, Jesus Christ Superstar is epic: it is narrated to us by Judas. However, it could also be dramatic (no framework of narration or perspective) or lyrical (a one-person show where Jesus sings about his own life). But these shows could all be told in any of the forms listed above, also making the category of narrative style secondary to the category of form.
Any musical then can be defined by the intersection of these various categories.
I’d say that’s a solid foundation upon which a discussion of genre and form in musical theatre can be built and delivered a seminar to that effect several years ago when I was reading for my Masters degree in Theatre and Performance. The terrain is wide and complicated and fraught with ambivalence and contradictory perspectives, but I think I’ve outlined above quite clearly how I feel it all fits together.
Musical Cyberspace 
Musical Comedy vs. Musical Play
Following my 2010 redux blog on “Genres of Musical Theatre“, I received a message from Hans Anders Elgvang, a regular visitor of this site, asking: ‘What are, in your opinion, the differences between the form categories musical comedy and musical play?’ This is my response.
The basic intention behind musical comedy is entertainment (Anything Goes, Crazy For You), while that of a musical play is enlightenment (South Pacific, West Side Story).
In terms of the book, musical comedies tend to present situations that are broader in their construction (Girl Crazy, Babes in Arms), while musical plays tend to be more subtle and detailed in their construction (The King and I, My Fair Lady). This applies to narrative as well as character (compare Annie and Frank in Annie Get Your Gun with Julie and Billy in Carousel). In musical comedies, improbabilities are overlooked (Nina changes her whole system of beliefs based on a piece of clothing in Silk Stockings), while in musical plays they are inexcusable (it is not a piece of clothing that shifts Mother’s ideology in Ragtime). Comedy in musical comedies is maximal, but musical plays use comedy within the limits of character: compare the one-liners given to Kate in Girl Crazy with the portrayal of Ado Annie in Oklahoma!
Although by no means a hard and fast rule today as it was in the past, musical comedies tend to be feature more contemporary settings (Anything Goes, Silk Stockings), while musical plays tend more frequently to be set in the past (Carousel, Oklahoma!): this characteristic of the difference between the two forms was particularly clear during the “golden age” of musical theatre and the delineation has been broken down dramatically in the past 40 years. Even so, when the past is depicted in musical comedy, it is often through the lens of contemporary sensibilities.
In terms of music and lyrics, the dramatic integration of numbers in musical comedies becomes flimsier the further one retreats into musical theatre history, with musical comedies from the mid-1940s onwards reflecting to some extent the sense of integration demanded by the musical play, where the dramatic integration of the numbers is always first and foremost. Musical comedies tend to have lighter content than musical plays, which tend towards more emotional content. A number in a musical comedy may move around from show to show, but this method of integrating material into musical plays is debatable depending on the example.
In terms of dance, musical comedy gives us “hoofing” (Anything Goes), while musical plays give us “choreography” (West Side Story). The difference is that the former involves a language devised for pure entertainment, while the latter involves a language that extends the action of the musical on a larger scale.
When modern versions of the musical comedy surface, making it difficult to distinguish between the two because of how musical comedy has had to change in reaction to the appearance of the musical play, one should return to the fundamental principle that separates the two: a musical comedy primarily entertains and a musical play primarily enlightens.