When I was younger, science fiction was pretty simple. In my world, you had space travel, and you had time travel. Astronauts, aliens, time travelers - there wasn't much else. Other forms were out there in various forms, but that's what it seemed like to me.
Nowadays there are almost as many sub-categories of the genre as there are genres of fiction. Scores of new sub-genres have sprung up over the years and obscure ones have expanded to the point where science fiction isn't even the over-arching category. Really it's all speculative fiction, which encompasses science fiction, fantasy, horror, alternate history, utopian, dystopian, cyberpunk, apocalyptic, post-apocalyptic, the paranormal, superhuman or superhero, and anything which touches on things (so far) outside the realm of possibility, including those with fantasy and horror elements.
Straight science fiction, as I've heard it explained, is comprised of two basic elements. Science or scientific principles must be key to moving the plot forward, and the basic underlying theme is one of humanity vs. technology.
Of course, there are many varied schools of thought on this, and many different definitions of what comprises science fiction, what its elements are, and how it is defined. Pick your favorite. They're equally valid.
I like my definition, because it's simple, and because it strikes to the core elements of the genre. Because of this, it helps to define it more clearly in the reader's or viewer's eyes.
The Terminator demonstrates this school of thought nicely, and coincidentally also falls into one of those two categories of my childhood - time travel. Science is a definite plot vehicle in this story, because without time travel - one of the key scientific elements - the plot is nonexistent. Even more broadly, time travel and the existence of sentient technology are both vital elements of the plot.
Secondly, the underlying theme of the entire story, from the first movie through the last movie or television series, is one of man vs. machine. It calls into question our self-destructive relationship with technology and provides a worst-case scenario of that relationship gone terribly wrong.
Many more of the classics can be viewed the same way. Philip K. Dick's Do Androids Dream of Electric Sheep?, or the more widely known movie adaptation, Bladerunner is one such story. If it wasn't for androids and their attempts to return to earth from Martian colonies, there would be no plot. And this story more so than almost any other speaks directly to the contradictory relationship between humanity and technology.
The speculative fiction genre has exploded in every direction. Bruce Bethke and William Gibson put cyberpunk firmly on the map. Space westerns and stories of space colonization are becoming more and more common following such stories as Joss Whedon's Firefly and Serenity, and the more recent Avatar and Cowboys vs. Aliens, and the reboots of the classic Planet of the Apes movie series. Virtual reality remains a common element, with movies like The Matrix and sequels, and the Tron reboot continuing to expand the sub-genre.
One of the keys to speculative fiction is that while it must remain fresh and believable, it is very perishable in nature. A story that seems new and postmodern at its debut seems antiquated and outdated after a few years of real technological advances. If you're looking for proof of this, dig out that old video cassette recording of Lawnmower Man, or watch some of the 1950's science fiction reruns. See what I mean? Archaic!
To a science fiction author, this means keeping on the cutting edge. It means constantly struggling to keep up with the latest moves in technology, and trends of where society is heading. It means consistently updating manuscripts and rewriting outdated material with subsequent edits. It's a tough job, but a fun one. And with so many different and exciting possibilities in the world of speculative fiction, it's one I wouldn't trade for the world.