An opera is primarily sung: the drama is carried continuously through arias, recitatives, and choruses, with music at the forefront. A musical typically intersperses songs with spoken dialogue, so plot often advances in the spoken scenes between numbers, and dance sequences are more common.
Classical Music notes another practical contrast: opera singing developed to project over an orchestra without microphones (often with more vibrato), while musical theatre is a newer form that usually uses amplification. Boundaries can blur—some musicals are nearly sung-through—but the usual distinction is continuous, music-led storytelling versus song-and-dialogue theatre.
Source: What is the difference between a musical and an opera? — Classical Music