As a subgenre of fantasy, Science Fiction (or SciFi, as most of us call it,) is actually a broad genre on its own. There are very few hard and fast rules for this subgenre. It can be far in the future, like Battle Star Galactica, or even a “long, long time ago”, like Star Wars. The only real factor is technology or a not-yet-realised scientific future.

Some SciFi is very pointedly a story that takes place in space (any space that’s beyond the Earth’s atmosphere, really) – think Star Trek or Alien – but others may use Earth or a planet like Earth to give the story a more subtle SciFi angle – Æon Flux and Real Steel can give you a bit of insight on this angle.

If you have looked at some of the older SciFi, you will even see that some of the elements that were Science Fiction in those days, are now a reality – like the Internet and nano-machines.

Popular themes:

• Space exploration
• Intergalactic politics with alien races
• Aliens
• Bio-engineering
• Nanotechnology
• Reviving extinct species (think Jurassic Park)
• Colonisation of other planets/solar systems
• Space pirates
• Bionics
• Robots
• Cyborgs
• Artificial Intelligence (AI)
• Domination of the human race by aliens/machines
• Mystical forces from space (2001: A Space Odessy, Avatar, Star Wars  and many more)
• Alternate realities
• Mutliverse
• Time travel
• Teleportation
• And lots, lots more…

It’s really so broad, one could write volumes about this subgenre. But, for me, it is important to note that it can easily be used as a flavour in the other fantasy subgenres. Examples of fantasy that typically contain SciFi elements are: Steam Punk, Cyberpunk, Dying Earth and Other Planet.

Never for a moment think that subgenres are mostly found in their pure forms. The very way they have found their way into existence is by developing from other forms of the main genre. The rules are hardly ever set in stone.