Posted by Soya in the "other" pile 13 years ago
Well, first of all, what is it? For many sf writers the term 'hard sf' is a little like that embarrassing tattoo you got in high school and just can't seem to live down now that you have a real job. Hard sf devotees tend to use the term in regrettably silly ways . . . like to talk about how 'hard' they are, and to weed out people they think aren't 'hard enough' to play in their league. In return, people who write the various subgenres of SF that don't generally include multipage explications of imaginary technology complete with equations tend to respond with equally absurd claims that there is no such thing as hard SF and the whole concept of hard SF is just the science fictional equivalent of a No Girls Allowed tree house. As a result, the hard sf label (actually coined in the 1950s by Astounding Stories editor P. Schuyler Miller) often evokes the worst excesses of 1950s Agenda SF (1) . . . stories where aliens were enemies, women were green, and white men saved the free world with science. All of which has nothing whatsoever to do with the nuanced and complex stories being written by today's best hard SF practitioners.
Check the Wikipedia:
gorilla: Hard science fiction, or hard SF, is a subgenre of science fiction characterized by an interest in scientific detail or accuracy. However, there is a great deal of disagreement among readers and writers over what exactly constitutes an interest in scientific detail. Many hard SF stories focus on the natural sciences and technological developments, but many others leave technology in the background. Others contend that if the technology is left in the background it is an example of soft science fiction. Another distinction within the genre revolves around portrayals of the human condition. Some authors seek to reflect technical accuracy within an advanced, nearly utopian society in which mankind has attained victory over most human ills; while others seek to portray the impact of technology on the human race with human defects still firmly in place and sometimes even magnified.  
Some authors scrupulously eschew such implausibilities as faster-than-light travel, while others accept such plot devices but focus on realistically depicting the worlds that such a technology might make accessible; the hard SF writer is permitted to foresee the automobile provided that he also foresees the traffic jam.  
In hard science fiction, the main characters are usually working scientists, engineers, military personnel, or astronauts. Character development is often secondary to explorations of astronomical or physical phenomena, but some authors foreground the human condition or the idea that individuals will have different values and ways of life in future societies where technological and economic circumstances have changed. Even in such cases, however, a common trope of hard SF hinges the resolution of the plot on a technological point.  
Hard science fiction writers usually attempt to make their stories consistent with known science at the time of publication (which also means that to later audiences their knowledge may be obviously incomplete; some older works depict astronauts walking on Venus in street clothes). Even when writing hard SF set in alternate universes where different physical laws apply, authors still attempt to create an internally consistent set of physical laws.  
jtown: That's a whole lot of words to say that hard SF deals with the science and soft sf deals with the social angle.