In comparison to other theatrical forms like operas, musicals use various genres of popular music and popular singing styles. Today, large theatres staging musicals generally use amplification of the actors' singing voices in a way that would generally be disapproved of in an operatic context.
While an opera singer is primarily a singer and only secondarily an actor who rarely needs to dance, a musical performer is often an actor first and then a singer and dancer. Someone who is equally accomplished at all three is referred to as a Triple Threat.
Unlike operas, musicals are almost always performed in the language of its audience. For example Les Miserables is originally composed in French but sung in English if performed in London.
However, for some works, production styles are almost as important as the work's musical or dramatic content in defining into which art form the piece falls. For example some of George Gershwin's works as well as light operas such as The Pirates of Penzance by Gilbert and Sullivan have received both, musical and operatic productions. Consequently, the overlap remains between lighter operatic forms and more musically complex musicals.