This list, much like the Fantasy list, is not exhaustive, nor is it definitive. Many people will disagree with some of these definitions, and we welcome your comments and your feedback. I'll start with what is arguably the most popular and go from there (though the whole list is not ranked by popularity - I'd pull my hair out trying to figure that out).
Again, in case you missed, we have a visual representation of most of the major genres in fiction.
SUPERHEROES - Once relegated to nerds-only comic books, superheroes have evolved into what is probably the most popular version of science fiction in any medium. Superheroes are created in a multitude of ways: via scientific experiments (some deliberate, some gone awry), from outer space, through genetic mutation. Some come from fantasy elements (like being birthed from the gods), but since they all utilize pretty far-fetched technology, you can pretty easily put these all in science fiction. I don't know that I need to give examples, but X-Men, Superman, Wonder Woman, and The Avengers are all very popular examples.
APOCALYPTIC - These stories focus on the failing of society. I think post-apocalyptic stories are the more popular ones here, but stories taking place as the world falls apart are pretty consistently popular as well. Pandemic (The Andromeda Strain and a million different zombie novels), super ecological disasters (Ashfall, The Day After Tomorrow) or war can cause the fall of civilization. But apocalyptic stories all deal with how one character or group of characters survive the disaster. Post apocalyptic stories are about people surviving in the aftermath (I Am Legend is a good example of a pandemic that collapsed society, and one man is trying to survive in the aftermath).
DYSTOPIAN - Closely related to apocalyptic, and often set in post-apocalyptic settings, dystopian is defined by the political tone of the story. "Dystopia" is the opposite of "Utopia," meaning everything has gone wrong. These books feature heavy, oppressive governments, and usually center on a character or group of characters trying to overthrow the government. Dystopians have been popular for a very long time, even though the term itself is fairly new. Some of the most popular recent dystopians have been young adult novels - The Hunger Games, Divergent, Matched - but the genre was popular in adult fiction long before the post-Hunger-Games explosion - 1984, Brave New World, Fahrenheit 451, Clockwork Orange. Dystopians have been declared "overdone" in recent years, but can also be overlapped with other sub-genres (Delirium - biopunk, Enclave - apocalyptic, Atlas Shrugged - thriller) and even be divided further.
TIME TRAVEL - Sometimes it's via a time machine (Back to the Future, The Time Machine) and sometimes it's more of a preternatural ability (Tempest, Jumper, Pathfinder). Either way, these stories are usually classified as science fiction. Sometimes they function like a portal fantasy (The Time Machine) and sometimes they don't. There's obviously some overlap with fantasy (particularly urban fantasy) and historical fiction. Time travel is a sub-genre that I think every science fiction writer says, "Yeah, I'd like to try that," and then the execution turns out to be murderously difficult and most of us give up.
STEAMPUNK - Ah. The genre I am asked to define most often. The easiest definition is "robots + corsets = steampunk." And that's correct. But it's also a wildly overly simplified definition. Steampunk is set in a time before we had widespread access to electric technology. The world is then re-imagined as if steam had remained the power of choice and technology continued forward as such. Sometimes it is a wholly fantastical world, as in The Parasol Protectorate series (aka Soulless, et al) or The Infernal Devices (aka - Clockwork Angel, etc.) Sometimes it is an alternate history, based in true events or with true or could-be-true characters like Leviathan or Wild Wild West. Sometimes it's more subtle, like The Prestige. The setting can be European or American, though it is rarely other. (Somebody get on that, please)
Also, old science fictions are being reassigned to this sub-genre, too. Jules Verne wrote everything that we are basing today's steampunks on.
CYBERPUNK - This doesn't get labeled very often, but with the emergence of steampunk's popularity, people are starting to put labels on cyberpunk fiction. And cyberpunk fiction is incredibly popular, and has been since the invention of cybernetics. These stories are stories of cybernetics run amok. Think The Matrix, BladeRunner, Minority Report, and (I say) Ready Player One. The internet rules the world, and is often the government/institution that needs to be taken down or escaped.
-PUNKS - There are a number of "punk genres" out there, in addition to the two most popular already discussed. Dieselpunk is usually set after World War I using diesel as the main power source instead of steam (The Rocketeer, Sky Captain and the World of Tomorrow). Coalpunk and Atompunk are both proposed sub-genres, though there are not many stand-out mainstream examples of either. Biopunk is more common and deals with genetic manipulation and biotechnology (Gattaca, and - to a lesser degree - Maximum Ride: The Angel Experiment)
MILITARY SCI-FI - This is exactly what it sounds like. A military or war story set in space or in a futuristic science-fiction world. Hammer's Slammers, War of the Worlds, and Ender's Game are some popular examples.
SPACE OPERA - These are dramatic stories in which the setting of space is relatively unimportant to the story. The story could take place anywhere, anytime, and have basically the same impact on us. Academy 7 is a good recent example. A LOT of people will say the term "space opera" applies to the scope of the story, sort of making Space Opera the science fiction equivalent of Epic Fantasy. If this is your definition, then Star Wars and Star Trek go here. The emphasis in Space Opera will always be on characters and plots, with the setting being secondary (even if it is detailed and perfectly foreign to us.)
SPACE WESTERN - Sounds contradictory? But it really works. The most defining characteristic of the western is the Rogue Cowboy, yes? Put him in space and you have a Space Western. Shoot outs, bandits, anti-hero protagonists, intergalactic saloons. Han Solo was sort of a space cowboy, and Joss Whedon's Firefly is probably the most popular example of recent years.
Again, as with fantasy, it's impossible to define every science-fiction exactly and place them all neatly into these categories. Time travel and superheroes can overlap, or time travel and steampunk. Space opera could be overlapped with military sci-fi, and space westerns could easily be merged with cyberpunks.
What do you think? What did I miss? What did I get wrong? Do you read science-fiction? Do you write science fiction? What is your favorite kind?