Difference between Space Opera and Science Fiction?

A while back a guy came into the place where I was working. He had a book (I don't remember the name). The book was, according to him, SciFi. I told him, "Great! My favorite SciFi is the Skylark Series by EE Doc Smith!" He sort of tsked this, saying, "Oh, that's only space opera." I really didn't know what to think of what he said. The funny thing was, I remembering looking at his book thinking, what makes his book SciFi any more than mine.

I started thinking about this incident the other day and am once again curious:

What is the difference between Space Opera and Science Fiction? What identifies each genre specifically?

NOTE: I had originally asked this here on meta due to the closed question: What is the relationship between fantasy and science fiction? which is very similar to my question. It was subsequently moved to the main site.