A rock opera is a rock music album or stage production that resembles the form of an opera. The historical reference of the word "opera" would most always include the live performance of such an event. In a true opera, the lyrical story would be told inclusively within the musical dialog. If the performance includes dialog between musical passages, it would in fact be an operetta.
It differs from a conventional rock album, which usually includes songs that are unrelated to each other in terms of storyline. However, the rock opera style overlaps considerably with concept albums and song cycles. More recent developments include metal opera and rap opera (sometimes also called hip-hopera). The category a particular work falls into is largely defined by the intent and self-definition of the work by its creator. The formal distinction may be that the rock opera tells a coherent (if sometimes sketchy) story, often with first-person lyrics sung by characters; while a concept album or song cycle sets a mood or maintains a theme. Some albums share characteristics of more than one category. Tommy, one of the best known rock operas, also had a rock musical production.
On a technical note, the phrase "rock opera" is, in terms of both music and theater, a misnomer. Opera consists of individual singers acting out a specific character within a drama, whereas rock opera is comprised of singers who sing a story, but do not act it out. The singers in rock opera can sing as a specific role, but this relationship is less rigid than traditional opera; singers rarely maintain the role for the entire album, usually assuming a different role for different songs, or simply narrating via third-person. Ironically, the phrase "rock oratorio" would be far more appropriate (despite the fact that oratorios are usually sacred in nature), as most of the conventions within rock operas are shared by oratorios.