A space opera is a work set in a far future space faring civilisation, where the technology is ubiquitous and entirely secondary to the story. It has an epic character to it: The universe is big, there are lots of sprawling civilizations and empires, there are political conflicts and intrigues galore. Frequently it takes place in the Standard Sci Fi Setting. In perspective, it is a development of the Planetary Romance that looks beyond the exotic locations that were imagined for the local solar system in early science fiction (which the hard light of science revealed to be barren and lifeless) out into an infinite universe of imagined exotic locations.
Space opera has a lot of romantic elements: big love stories, epic space battles, oversized heroes and villains, spectacular places.
Note that this is quite different from the original definition of space opera, which was a derogatory term. It was a variant in a long line of terms for substandard genre fiction: ‘horse opera’ was bad Western fiction, whereas a ‘soap opera’ (so named because they began as hour-long ads for soap) was a hackneyed drama. The phrase was coined in 1941 by Wilson Tucker to describe what he called “the hacky, grinding, stinking, outworn space-ship yarn”. Weirdly, this means that today many works which were originally touted as examples of ‘serious’ science fiction, such as the Lensman series, are today held up as prime examples of Space Opera. As more authors and writers came to embrace the space opera style, the term has largely lost its negative connotations. Assisted by writers who regarded all tales of action and adventure in space as bad, and so tried to label it all “space opera” in pejorative sense; they succeeded with the label, but not with keeping it pejorative.
Star Wars is probably the most famous modern example of space opera. In Star Wars, technology is either magic (the Force) or slightly faster versions of today’s gadgets (blaster rifles, hovercars, space ships) and the characters would be right at home in a fantasy novel (evil emperor, farmboy, princess).
The opposite of Space Opera is Hard Science Fiction.