Space opera is a branch of the science fiction genre. It's distinguished from “hard” science fiction, by its focus on adventure, melodrama, and romance in a science fiction setting (according to wikipedia, which actually is a very good source of information for all things geekish).
Space opera is the fun side of science fiction. This is where heroes leap into tiny space ships and hurl themselves at faster-than-light speeds to land another star system before lunch. This is where space fighters zoom through the air, dodging each other like 1940s Messerschmidts and Spitfires. This is where heroes leap from floating city to floating city, sometimes zapping bad guys with their laser gun, sometimes fencing with a medieval long sword.
The great part about space opera is how much fun it is. I remember as a boy, pouring through Edgar Rice Boroughs's John Carter, Warlord of Mars books and absolutely loving them. They were pure adventure, where a man from Earth magically appears on the surface of the dying Mars and eventually becomes its greatest champion, marrying the voluptuous space princess Deja Thoris. Of course, once they were married, our hero spent the rest of the books rescuing said princess from the nefarious forces of evil. Ahh, such is the genre; nobody ever has a relaxing day. Everyone's just so busy fighting the forces evil.
I think space opera is a genre that's there as fantasy fulfillment. It exists as a world for derring-do, for heroes to be heroes. The trouble is when people start taking space opera seriously as real science fiction, which is where Star Wars come in. Science has little bearing in the worlds of George Lucas, with a couple of exceptions. The world of Luke and Leia is one of adventure, where evil is always lurking close by (or at least a nearby star system), waiting and planning. When I was young I absolutely loved space opera.
But of course that changed. It's not that my taste gradually got more sophisticated. I still love sitting down in front of the TV and watching The Empire Strikes Back with my six year-old son. What happened is that I personally got tired of a universe set up specifically for the adventure. I learned that when there's a lot of violence, there's also blood and death (and I don't know about you, but death just takes away from the fun of a good battle). I learned that it takes a lot of farmers to support a few aristocrats, and the farmers' problems might be more real than any warrior born from nobility.
That's what makes a science fiction movie like Wall-E so successful. It's about a very common robot doing something very common, collecting garbage. When he meets the robot from the future, Eve, he's smitten. And even Eve isn't so uncommon. She's built to be a tough probe droid, with one arm functioning as a powerful laser cannon, but her slick iPod looks are really just a function of when and where she was built. These two electronic protagonists then discover that despite Earth's pollution, it can still support life. The rest of the movie is a struggle against the vested interests who want to keep the remains of humanity shut up in a large space ship.
Now Wall-E is a kids' movie at heart, so it breaks certain conventions of reality that might be a problem in a more sophisticated movie, but it does deal with the very real issue of our planet getting more and more polluted. The adventure comes out of that, as opposed to space opera where the entire universe seems structured around adventure.