The lines between those two forms are vaguer than ever these days. One can generalize about opera being sung-through and musicals requiring spoken dialogue, but there are tons of exceptions on both sides! People talk about the plots of operas being more intense than musicals, but that doesn’t hold water anymore — musical theater is capable of devastating dramatic power. I think the best we can do is to speak about the kind of singing
that’s required in opera — there’s an amplitude and vocal size, a breadth and warmth of tone, an extensive range, and a degree of flexibility, that all speak “opera” to the ear. That kind of voice isn’t needed for, say, Avenue Q
or The Book of Mormon
! Musical-theater voices aren’t asked to sing unamplified over large orchestras — and in fact, orchestras in musical-theater productions nowadays are a fraction
of the size of, say, a Wagner or Strauss orchestra.