Space opera?
Well, there's probably a strict set of rules somewhere, but set genre specifications are anal, so my definition is kinda loose. To put it simply, anything that focuses on the leaps on technology that have been in the future and tries to explain them scientifically is scifi. Space opera on the other hand is pretty much space fiction. No technology is explained as such, it just is, like lightsabers of hyperdrive.
Another example; Babylon 5 is very much scifi, while Firefly is space opera(or a cowboy story in space). Cowboy Bebop is space opera, most mecha series are space opera, Silver Surfer is space opera, so are Giraud's graphic novels and Dan Simmon't Hyperion, while on the other hand Star Trek is not.
Some people characterize space opera as epic, but it doesn't have to be, although a lot of the stories do have galaxy-wide happenings and grand themes,, like fight between light and darkness and revenge. Such, is space opera.