I hope to do reviews of sci-fi and fantasy stories professionally some day. In writing these reviews, I often define the subgenre of particular stories. As far as I can remember, I've never written a post describing the subgenres of sci-fi, but I have posted about fantasy twice. The more recent, far better post can be found here. Today I would like to detail just the four main types of sci-fi prose: hard SF, military SF, soft SF, and space opera.
Hard SF is also defined as technological sci-fi. This is the most common use of the subgenre. Hard SF emphasizes science, particularly the "hard" sciences or natural sciences. These include chemistry, biology, physics, mathematics, engineering, and others. Technology is not a required aspect to a hard SF story, though most stories of the subgenre deal with it. One story I'll be reviewing for class has its scientific bases in zoology. I consider the story hard SF (though it's a blend with contemporary fantasy). In hard SF, the facts behind the science are crucial; however, because these stories are often projected far into the future, the science only needs to be plausible. Analog Science Fiction and Fact is perhaps the most prestigious magazine for and most frequent publisher of hard SF.
Military SF is well-named. It's science fiction that focuses on militaries. Usually the protagonist is a soldier, though this is not a hard requirement. Military SF is fairly rare, though not as rare as one might think. Even in sci-fi stories that aren't strictly military, war and combat are frequent elements. Military SF exists on a spectrum from space opera to hard SF. Sometimes you get unlikely aliens and what seems like magic, while other times you get detailed explanations of the physics behind a soaring missile. When well-written, military SF gives us fantastic romps. As far as I know, there aren't any major publishers who focus on military SF. Stories in this subgenre appear sporadically.
Soft SF has many clarified names. It can be psychological SF, philosophical SF, or sociological SF. The borders of soft SF are blurry. Soft SF is supposed to be at least somewhat focused on science, most typically the "soft" sciences or social sciences. The humanities can also lend themselves to soft SF. It is unclear where stories with little "soft" implications and some "hard" implications should be placed. Most of those stories will end up being either military SF or some blend of sci-fi and fantasy, but that is not always the case. Soft SF seems to be attributed to stories less often than the other main subgenres of sci-fi. Sometimes stories in the gray region are coupled with true soft SF under the nondescript name of "sci-fi." True soft SF is the sort of stuff that makes you question what you know about yourself, society, and even existence. It can show you what's inside your head, describe society better than most literary fiction, or present a deep philosophical question. Orson Scott Card's books Speaker for the Dead, Xenocide, and Children of the Mind (sequels to Ender's Game) are excellent examples of soft SF.
Because it hasn't really changed since I wrote about it before, I'm just going to quote my post about fantasy subgenres to describe space opera, since realistically it's a blend: "Space opera is where the line between fantasy and science-fiction blurs. Some consider it fantasy, others science-fiction. Star Wars and Star Trek are both considered space opera. Science as we know it is completely disregarded on multiple planets in both settings, which some argue makes space opera a subgenre of fantasy. The often futuristic settings of space opera stories make them appear to be science-fiction. To be fair, space opera can be regarded as a subgenre of both science-fiction and fantasy."
Science fiction has far fewer subgenres than fantasy. Part of the reason for this is that fantasy is a much larger concept. The other part is that the boxes we have built for sci-fi are quite large. Unfortunately, it's pretty difficult to break these subgenres up into better classifications, other than to separate them out into biological SF, sociological SF, etc. This wouldn't actually help very much because stories that blend would still blend. My rule of thumb is that if I can't put a subgenre to a story I don't; I just call it "sci-fi" and leave it at that.