This article is from the Classical music FAQ, by Gabe M. Wiener gabe@panix.com with numerous contributions by others.
Generally, a musical has dialogue with interspersed songs. Opera is
generally sung through, the dialogue portions being replaced with
recitatives (music which is intoned in a way that resembles speech).
There are notable exceptions to this rule, e.g. Carmen (Bizet) and The
Magic Flute (Mozart), both of which have spoken dialogue. The German
name for operas with spoken dialogue is singspiel (pronounced
ZING-shpeel). German productions pre-Wagner were almost always
singspiel
Many musicals, such as Les Miserables, Chess, Joseph, etc. are sung
through, and are, in the classical world, often referred to as
"popular operas" or "rock operas" to signify that they do bear some
resemblance to "classical" opera.
Another important difference is that in musicals, the principal
singers also dance. In opera that never happens.
 
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