A few science fiction concepts

Science fiction is a complex beast, at least among those who take it seriously. There are a number of concepts that are taken for granted by most science fiction readers, and mystifying to anyone not a part of that culture. An understanding of these basic concepts is vital for anyone who might desire to write for a science fiction audience.

When writing science fiction, you are effectively conceiving a new world. Much like in fantasy, it is important that the rules of this world are consistent with themselves, even if they have little relation to the world we live in. Even a setting as seemingly obtuse as the one presented in The Hitchhiker's Guide to the Galaxy (by Douglas Adams) comes to an understanding with the reader about what sorts of things they can expect. To significantly shift paradigms after establishing a consistent setting is a risky maneuver, and may leave readers feeling betrayed. Most of these concepts involve the categories that a reader will place your setting into in order to determine what they can expect.

Scope:
How big is your effective setting? one country, one planet, one solar system, a cluster of stars, an arm of a galaxy, a whole galaxy, or even several galaxies are all possibilities. The scope of your setting will be tied significantly to the maximum distances and speeds that people can expect to be able to travel. If no faster than light method of travel is possible, then your setting will likely be relegated to just a few colonies, or even just a single solar system.

The scope of a setting tells readers what reasonably is at stake, and how complex and diverse your world is. If the furthest anyone is able to go is out to the asteroid belt, they will have a different set of options than if they were in a setting in which they could reach the next galaxy. If there are thousands of inhabited worlds, with billions of people on each of them, then the crush of it's vastness should seem overwhelming, especially if they all are in communication and travel range of each other.