Genre - The Difference Between a Symphony and a Song
There are many genres in music just as there are many genres in film. In film these could be action, horror, science fiction, noir, romantic comedy, mystery, and so on. In music we find symphonies, concertos, songs, suites, operas, and many more. Some genres are designed for large ensembles like orchestras and choruses, while others are for chamber ensembles (small groups) or soloists (individual players). Although there are many genres, some play a larger role in western music than others. The lessons on vocal and instrumental genres focus on the most common and influential genres in western music.
Genre vs. Form
It is sometimes difficult to distinguish between genre and form, but they are not the same thing. Form refers to the structure of the music on the page, while genre is based on the performing forces (voices and instruments), the purpose or function of the piece (dance, hymn, opera, etc.), the style used, and even the cultural or historical context of the work.
For example, an aria is a genre of solo song in opera used to communicate the inner thoughts of a character (like a soliloquy in a play). Arias are often in binary or ternary form (also known as da capo form), but could be strophic, through-composed, or follow any other form. However, if the same piece were not part of an opera its genre would no longer be aria, it would be art song.
Style and Sub-Genres - Opera and Rock Opera
Sub-genres are often defined in terms of style. For example, the genre of song has sub-genres such as rock song, polyphonic song, musical theater song, hymn, and so on. Another example is the relationship between opera and rock opera.
An opera could be defined broadly as a theatrical presentation (a play) in which the characters' lines are sung rather than spoken. The vocal style used in historical opera reflects the fact that before electronic amplification voices had to be big and loud so they could be heard in a large concert hall over the orchestra used to accompany them. A rock opera is simply an opera (sung play) that uses the style and instruments of rock music. Examples of rock operas are Rent by Jonathan Larson, Tommy by The Who, and Jesus Christ Superstar by Andrew Lloyd Webber and Tim Rice. The term 'rock opera' is sometimes used as a synonym for 'concept album' (such as My Chemical Romance's The Black Parade, The Beatles' Sgt. Pepper's Lonely Hearts Club Band, or Pink Floyd's The Wall), but only those with a clear narrative (a storyline with characters and events) that is told completely in song (no spoken lines) should be called rock operas.