Subgenres

Science Fiction is a broad, vast genre.  Although the most popular face of Sci Fi is space travel, there are dozens of other subgenres within the realms of Sci Fi-- all it takes is asking "What If?" Here are some of them.

Dystopias

Dystopian societies are often set in the near future, after some sort of catastrophic disaster (such as nuclear war, revolutions, environmental change and  overpopulation) that allowed whichever group is in charge to sieze uncontested power.  Advanced technology is used to control, observe, and threaten the population.  Descriptions of the society, political climate, and economic structure are crucial to the world building in dystopias.

Classics of the dystopian subgenre include Ray Bradbury's Fahrenheit 451, George Orwell's Nineteen Eighty-Four, and Aldous Huxley's Brave New World.  Dystopias are currently popular in Young Adult fiction, such as the Hunger Games trilogy by Suzanne Collins, the Uglies trilogy by Scott Westerfeld, and the Divergent trilogy by Veronica Roth.

Cyberpunk

The cyberpunk subgenre solidified in the 1980s, and features highly advanced technologies, uncaring neon-lit urban landscapes, and anti-hero protagonists.  Stylistically, it is similar to the hard-boiled detective novels and film noir of the 1930s and 40s, with loners and criminal elements operating outside mainstream society.  Dystopias, corporate rule, cyborgs, hackers, cyberspace and the internet are common elements.

Neuromancer by William Gibson is one of the seminal works of the genre.  Other important titles include Snow Crash by Neal Stephenson, the Mirrorshades anthology edited by Bruce Sterling, and Synners by Pat Cadigan.  Ghost in the Shell and Akira are relevant anime series, and movies like The Matrix and The Thirteenth Floor were heavily influenced by the cyberpunk movement.

Steampunk

This subgenre is primarily linked to alternate histories of the early Industrial Revolution era, when science was young and technology meant clockwork, springs and steam engines.  The aesthetic is paramount, and stories are commonly set in the Victorian or Edwardian years, or the American Wild West.  Airships, mechanical creatures and devices, early electricity (a la Nikola Tesla) and chemicals (such as in photography) often play a part.

Steampunk works often draw influence from Science and Speculative Fiction writers of the late 19th century, such as H.G. Wells, Jules Verne, Mark Twain and Robert Louis Stevenson.  The Difference Engine  by William Gibson and Bruce Sterling supposedly brought steampunk to the mainstream in the early 90s. Current popular titles include the Boneshaker series by Cherie Priest, the Leviathan series by Scott Westerfeld, and the Halcyon series by Joseph Robert Lewis.  Graphic novels such as The League of Extraordinary Gentlemen by Alan Moore and Kevin O'Neill and the webcomic Girl Genius by Phil and Kaja Foglio also fall under the umbrella of steampunk.

Military

 The military is a common theme in Science Fiction, for obvious reasons.  If technology and its applications are a crucial piece of what makes Science Fiction work, then who else would have the most advanced technology but the military?  It serves as an appropriate means for providing action, adventure, and themes of bravery and duty.  Wars can be between races, planets, and/or humans.

A classic example of military SF is Robert A. Heinlein's Starship Troopers. Modern series include Lois McMaster Bujold's Vorkosigan Saga, the Familias Regnant series by Elizabeth Moon, and The Lost Fleet series by Jack Campbell. A well-known YA title is Orson Scott Card's Ender's Game. More titles can be found here and  here.

Alternate History

Alternate History (also known as allohistory or uchronia) is a corollary genre to Time Travel stories.  There are subtle differences between plot devices that make a narrative either Time Travel or Alternate History.  Generally speaking, an Alternate History story must include a specific point of divergence from past history as we know it and also explore the results of that change.  It need not involve time travellers or a time machine (although it can); Robert Harris's Fatherland is a good example of an Alternate History that does not involve Time Travel.

Alternate histories that do involve time travel can involve going backwards in time and either changing or attempting to not change the past. Other stories involve a notion of multiple dimensions or timelines, or a person from a different timeline feels that things are somehow "not right" as a result of meddling.  Frequently these novels require a good amount of research and can read like Historical Fiction. Some Alternate History works are Making History by Stephen Fry, 11/22/63 by Stephen King, and the 1632 series co-written by Eric Flint (and others, by individual work). Award winning authors include Ian R. MacLeod, Harry Turtledove and Robert Conroy.  Here you can find other works of Alternate History.