Space Opera is a sub-genre that is usually set mainly in outer space and often deals with large scale events and themes. The definition has changed over time and used to represent really bad scifi, now though it's usually defined as dramatic, heroic and large scale with much of the story character driven (hence the term space opera, like soap opera). It's also probably the broadest and most popular category to fall into.
A word to the wise ain't necessary - it's the stupid ones that need the advice.
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