Hard Sci-Fi refers to a particular emphasis on scientific detail and / or accuracy. Stories based on real science & engineering for the most part, it is driven more by ideas than characterization. Near realistic science and technology are central to the plot. Stories in this broad subgenre depict technology that conforms to actual scientific knowledge and physical laws, or extentions of them that scientists consider plausible.
The sub-genres below overlap with Hard Sci-Fi, or have common shared descriptions: Firm Science Sci-Fi, Mundane Sci-Fi, and Near Future Sci-Fi. Resources: Wikipedia.org, imdb.com