Within Hard or Soft SF, there are different types, or subgenres, of science fiction. Each subgenre is a group of stories that uses similar ideas or styles of writing. Publishing companies and critics put works of SF into different subgenres to help describe the work to help readers choose which books to read or movies to watch. Assigning genres is not simple. Some stories can be in two or more genres at the same time. Other stories may not fit any genre.