Andrew Lloyd Webber's musicals are significantly different from 'golden age' musicals of the 40's and 50's. You call such pieces is rock opera, but this is probably not the most accurate term as it more often refers to a musical album with connecting narrative through the songs (and no element of staging). Thus, I prefer (and will be using) the term megamusical instead.
Here are the key differences between these different genres - opera, musicals, and megamusicals:
Opera:
- typically through-sung
- typically not electronically amplified
- typically part of the classical music tradition and singing style
- typically performed in original language (at least in the USA)
Musical theater:
- typically has substantial dialogue
- typically electronically amplified
- typically part of the popular music tradition and singing style
- typically performed in language of audience
Megamusical:
- through-sung (like an opera in this respect with little to no spoken dialogue)
- typically electronically amplified
- typically part of the popular music tradition and singing style
- typically performed in language of audience
In addition, megamusicals tend to also have the following characteristics that further differentiate them from other musicals:
- sweeping/epic in scope and story
- usually a historical subject or setting
- typically very dramatic and comedy has a relatively smaller presence
- complicated/expensive sets (often with a unique, novel feature like the chandelier in Phantom)
- generally has a significant marketing aspect (recognizable logo, advertising, etc.)
In short, a megamusical is bigger than a musical in just about all respects. Shows like Jesus Christ Superstar (probably the first megamusical), Cats, Phantom, Les Mis, et al. are all part of the musical tradition, but got a little closer to opera as they became bigger and grander. Of course, there are exceptions as these categories are not rigid proscriptions but rather descriptions of common trends.