Please note: The following is merely my opinion and most other Researchers are unlikely to agree with it. Please do not assume that these are agreed-upon conventions.

Most readers of creative fiction (fantasy, speculative fiction, science fiction, sci-fi) would agree that it falls into several classifications, and that they like stories from some of these classifications more than they like stories from others. Everyone, or even a majority of people, are not likely to agree with any one classification system, the purpose of this entry is to propose one such system.

The categories into which it is proposed that creative fiction be divided into are fantasy, speculative fiction, sci-fi/technological fantasy, soft SF, and hard SF. What follows is an explanation of each of the categories.

Fantasy

Fantasy is defined as those types of creative fiction that include stories that are set in situations completely unrelated to the real world. For example, magic, talking animals, and myths. This includes worlds like "The Lord of the Rings", all fairy tales, and a wide variety of other works. Since the main purpose of this entry is to define types of science fiction, fantasy is only mentioned here to differentiate it from science fiction.

Speculative Fiction

Speculative fiction is made up of of two types of fiction, future fiction and alternate timeline fiction.

Works of future fiction are set in the future but do not consider technology as central to understanding the future. A novel of future fiction might, for example, deal with a future that is technologically the same as ours, but in which sociology makes the future different. Works of future fiction do not have technology and changes in technology and scientific knowledge as the basis for their plots.

Alternate timeline fiction deals with how the world would be different if historical events had happened differently. For example, books set in worlds in which Napoleon won at Waterloo, Robert E. Lee was victorious at Gettysburg, Rome never fell, etc. are alternate timeline fiction.

Sci-Fi/Technological Fantasy

Sci-fi/technological fantasy (hereafter referred to as just sci-fi) includes to those fictional works that attempt to be science fiction but have little or no regard for the laws of physics and for an accurate understanding of the universe. Such works could generally be written in a non-science fiction backdrop. Sci-fi stories often involve worlds that are completely impossible, and may include descriptions of this or other solar systems that make no sense. For example, they may treat the asteroid belt as being very densely populated, so much so that it is difficult to travel through without hitting an asteroid every few seconds. They may ignore the lack of air in space or treat every world as habitable. In general, technology may be the basis of story, but the technology often breaks simple laws of physics. Works of sci-fi usually amount to fantasy stories that replace magic with technology, and the technology often gives characters similar abilities to those they would have in a fantasy book. Sci-fi also includes books and movies that are intended to be or imitate science fiction, but that are written by people with so little scientific knowledge that they are more fantasy than science fiction. Examples of sci-fi include Star Wars, most popular science fiction movie, although there are some exceptions, many books written by mainstream writers that are attempts at science fiction, and children's books that imitate science fiction are intended to be science fiction.

SF (Science Fiction)

Science fiction is defined as those works of fiction that consider technology central to the development of society that are set either in a future that is technologically different from the present, or set in an alien society that is technologically different from our current society or that differs from human society because of physical differences between humans and the aliens involved.
Science fiction is generally divided into two categories, soft SF and hard SF.

The term soft SF refers to those SF stories in which an effort is made to ensure that the science in it is correct, but in which technological differences are not as important to the plot as sociological differences. These stories may deal with completely alien societies, or with societies that are different from ours because of environmental changes in the future. Generally, the science involved in the plot is accurate, but minor scientific errors not pertinent to the plot are considered acceptable. Examples of soft SF include Isaac Asimov's short story "Nightfall" and the wide variety of stories set on earth after an asteroid impact or nuclear holocaust or other natural disaster. These stories are more involved with the sociology of the future than the technology.

The term hard SF refers to those SF stories in which technological change is the basis of the plot. In hard SF stories, no science errors are tolerated, with the following exceptions:

1.) Time travel is allowed, as long as an effort is made to justify it in terms of known physics and it is self-consistent.

2.) Superluminal travel and communication is allowed, as long as the author justifies it in a way that does not violate relativity and it is self-consistent. Several common methods for designing superluminal drives that don't violate relativity are:

A.) Hyperspace- Ships travel in a higher dimension that allows
superluminal travel, this space can usually only be accessed
away from a gravity field, explaining why we haven't
discovered it yet.

B.) Space warps- Space is bent in higher dimensions so that
ships can jump instantaneously between two points, for
example the mouths of a wormhole.

C.) Tachyons- Ships are somehow converted into theoretical
particles called tachyons that always travel faster than
light. (These particles do not actually violate special
relativity, amazingly enough.)

Hard science fiction stories generally involve plots that are based on changes in technology in the future and how they will affect society. Examples of hard science fiction include "The Mote in God's Eye" by Larry Niven and Jerry Pournelle, "The Dragon's Egg" by Robert L. Forward, and "Have Spacesuit, Will Travel", by Robert A. Heinlein.

Since scientific knowledge changes, hard science fiction stories often become out of date rapidly. The question is, does a story that is based on outdated science automatically become non-hard SF because it is based on a inaccurate assumption? The answer is no. An example it that, until the 1960's astronomers were certain that Mercury's rotation was tidally locked to the sun. Now, the opposite is known to be true. Still, stories written before it was realized that Mercury is not tidally locked should be considered hard SF even though this assumption is now known to be false.