In the basic sense, science fiction deals with science, as it is now or how it may be in the future. Merriam Webster Dictionary Online defines science fiction as “fiction dealing principally with the impact of actual or imagined science on society or individuals or having a scientific factor as an essential orienting component.” Beyond this basic idea, the genre includes many different types of stories and themes. Several widely acknowledged sub-genres exist. As with much the in science fiction and fantasy genres, the boundaries between these categories are often blurred and argued. Many works can fit in more than one sub genre.

Hard SF vs. Soft SF
Some readers and writers argue that a work is not true science fiction unless the science is not just necessary to the plot, but central to it. Hard SF focuses on “hard” science and is often written by authors with a strong scientific background. Generally works of hard SF have an emphasis on scientific detail and accuracy. Writers of hard SF include:

• Isaac Asimov: A prolific writer with a PhD in biochemistry, he is best known for his Foundation series and "The Three Laws of Robotics".
  
• Greg Bear: His work often features cutting edge science. His novel Darwin's Radio (1999) won the 2000 Nebula.
  
• Arthur C Clarke: One of the major names in science fiction, he is best known as the author of 2001: A Space Odyssey (1968).
  
• Larry Niven: Best known for his Known Universe books (including the Ringworld novels), his work often includes physics.
  

With Soft SF, the focus is more on the “softer” sciences, such as sociology, psychology, and philosophy. These stories study the effect new sciences have on people and society. Some examples of soft SF include:

• Ray Bradbury: His pastoral style is known from The Martian Chronicles (1950) and Fahrenheit 451 (1953).
  
• Ursula K LeGuin: Her most well-known science fiction title, The Left Hand of Darkness (1969) explores identity issues through a hermaphrodite alien race.
  
• Lois McMaster Bujold: She is best known for her space opera, the Vorkosigan saga.
  
• Frank Herbert: He is best known for his career-spanning Dune universe.
  

===>Part Two: Technological Scifi
===>Part Three: Military, Apocalypse and Aliens
===>Part Four: Alternate History, Time Travel and Humor