Alternate History
Alternate Histories explores a history that has diverged from our own and developed differently. Some of these works may use the idea of a mulitverse (a hypothesis that multiple possible universes exist) and take place in a parallel world whose history mirrors our own. Some examples of alternate history include:

• The work of Harry Turtledove, by far the most popular author of alternate history
  
• "The Sound of Thunder" (1952) by Ray Bradbury: A careless time traveler steps on a single butterfly, thus changing the future.
  
• The Man in the High Castle (1962) by Philip K Dick: An Alternate world where Germany and Japan won World War II.
  
• The Plot Against America (2004) by Philip Roth: FDR is defeated in 1940 and Charles Lindbergh wins, leading the nation into fascism.
  

Time Travel
Time travel stories center around the ability to travel through time and the effects it has on the traveler or the future. Some examples of time travel include:

• The Time Machine (1895) by HG Wells: The classic story of a man who builds a time machine.
  
• 1632 (2000) by Eric Flint: A six-mile sphere in West Virginia is transported to Germany during The Thirty Years War.
  

Humorous
This sub-genre is not defined so much by theme, but by its tone. Some examples of humorous science fiction include:

• Hitchhiker’s Guide to the Galaxy by Douglas Adams: When the Earth is destroyed to make way for an hyperspace bypass, Arthur Dent and Ford Prefect hitch a ride and begin an adventure across the galaxy.
  
• Phule series by Robert Asprin: The series focuses on a misfit group of troopers in the Space Legion.
  

<===Part One: Hard Scifi vs Soft Scifi
<===Part Two: Technological Scifi
<===Part Three: Military, Apocalypse and Aliens