Just as 'opera' evolved over the centauries one could say that 'rock opera' is a further evolution.
Just like 'opera' it tells a story but a very modern setting, with rock instruments and instrumentation, with 'rock' vocals.
Factually, you could take an 'opera' and update it by transcribing the music for 'rock' instruments etc and update the setting / story to modern times.
Think of Shakespeare's Romeo and Juliet and what have you got? A story written in 1590's and adapted numerous times - think of Bernstein's 'West Side Story' that totally 1950's.