A space opera is basically an action adventure in space, usually involving interstellar travel, space battles and so on. EE 'Doc' Smith's 'Skylark' books are among the earliest space operas. Gordon Dickson's Dorsai series, Larry Niven's Known Space books and the Man-Kzin Wars anthologies are also space operas. Asimov's Foundation books Iain Banks Culture series and Brin's Uplift series broadened the scope for the sub-genre. Current writers like Peter Hamilton, Ken MacLeod and others continue to tell stories that can be broadly classified as space operas for the emphasis on interstellar action and conflict.
While 'space opera' began as a derisive tag for simplistic action sagas that happened to be set in space, the sub genre has since evolved into a complex, varied thing, perhaps most analogous to the epic fantasy strain in fantasy fiction.