Space Opera got its name back in the Thirties and Forties, when seriously cerebral intellectuals did not want their explorations of possible technology lumped in with mere adventure stories, where rayguns of uncertain principle and starships that could travel across the Galaxy in a week took the place of six-guns and stagecoaches, while the derring-do continued unabated.

Named in parallel with the horse opera (Western), this is usually Science Fantasy, where the derring-do is lots more important than the plausibility of the gizmos used. This is especially used for the story from another genre transferred to an alien setting, like a Western transferred to a colonial planet. The shootouts use ray guns instead of six-guns and the heroes ride rocket sleds instead of mustangs. Star Wars springs instantly to mind, along with Buck Rogers.

A good space opera will outsell hardcore SF nine times out of ten. This could also have had its effect on the tone in which the genre name is mentioned. Many of us love it when it is well done: Edgar Rice Burroughs was a master of it, and the original 'Star Wars' is a wonderful distillation of the genre.

If the story and plot and excitement matter more than just how the science works, you've got a space opera -- more power to you. This field overlaps heavily with the science fiction romance, since the entire motivation for many a space opera comes from trying to win the love object of the protagonist.

• Hunters of the Red Moon by Marion Zimmer Bradley
• The Survivors by Marion Zimmer Bradley and Paul Zimmer
• The Barsoomian trilogy: A Princess of Mars, The Gods of Mars and Warlord of Mars by Edgar Rice Burroughs, as well as the other books of the series:

Thuvia, Maid of Mars
The Chessmen of Mars
The Master Mind of Mars
A Fighting Man of Mars
Swords of Mars
Synthetic Men of Mars
Llana of Gathol
John Carter of Mars

• Star Wars, The Empire Strikes Back and Return of the Jedi by George Lucas
• The Starship series by Mike Resnik: Mutiny, Pirate, Mercenary, Rebel, Flagship
  
• The Demon King novels by Jack Vance, including:
  The Star King
  The Killing Machine
  The Palace of Love
  The Face
  The Book of Dreams
• Star Trek and all of its spin-off novels