Opera is when a tenor and soprano want to make love, but are prevented from doing so by a baritone.
Though it might seem like a silly question, what exactly is an opera? There's no easy answer. Michael Leiris in his work Operatics defines opera as, "any theatrical work whose basic medium is song. A wide variety of genres fall under this definition: opera strictly speaking, comic opera, light opera, operetta, musicals, lyric drama, etc." The common conception of opera might be fat women in horned helmets but the reality is it's an art form not all that easily pinned down. Il Trovatore is an opera by all accounts but what about Porgy and Bess or Sweeney Todd?
It's a difficult path to tread to elevate opera above operettas and musicals since the forms aren't readily separated. There are probably more objective differences between Birtwistle's The Minotaur and Puccini's La Fanciulla del West than the latter work and Lloyd Webber's Phantom of the Opera, yet the former two are commonly called operas and the latter a musical!
All this said, "opera strictly speaking" is not well understood, so we'll start with them!.