The word "culture" sticks out to me. In almost any context where I see it, it makes me curious, and makes me want to comment.
So what is culture?
Well, whole classes have been dedicated to this topic, as you might imagine. Probably one of the first things that comes to mind is "high culture," what we mean when we say someone is "cultured." Art, music, theater, etc. The finer things in life. That's certainly one of its meanings, but it only captures the tiniest part of what culture really is.
Culture is what we do.
I like to think in terms of what's called "cultural practices." These are the special things we do that form a part of our routine, our habits, etc. The way we interact verbally involves cultural practices. Our sense of objects and how we relate to them.
Whenever we do anything, we are enacting our culture. We aren't contained by culture. In a thread some time ago on the Analog forum, someone mentioned The Force from Star Wars - I loved the analogy. The Force is all around us, it is in us, etc. Culture is more interesting than The Force, though, because by enacting it, we pass it on to others, and simultaneously we bring about change in it.
Culture is a quality of interaction - not a written set of rules that people have to follow, but a way of doing things. We can articulate the rules, and sometimes we've been taught them explicitly, but we don't just follow them - we hold a relationship with them. We discuss them perhaps, or rebel against them, or value them, or defy them, or cherish them...
They're like the road we walk on. We can choose to follow the road to its destination, or we can walk away from the destination. But leaving the road entirely is far more difficult and dangerous.
When you think of culture in terms of interactions and cultural practices, it becomes far easier to grasp what people mean when they talk about "a family culture" or the culture of a smaller group. For every group that engages in regular interaction, a set of conventions will emerge through that interaction. Thus we can have "football culture," enacted by a group that meets in association with football events. We can have "company culture," enacted by the members of a company. An online forum can have a culture, too - witness the online discussions regarding the difference between the Analog forum and its neighbor, the Asimov's forum.
At least one of the consequences of this conception of culture is interesting for writing in sf/f. The idea is that, since we enact culture in everything we do, any smallest piece of interaction that you capture will contain evidence of that culture. To put it in writing terms, the culture of an alien world, a future Earth colony or a fantasy society will show itself in every single scene - and in every part of that scene, and in everything its people say, and in every object they possess, and in every attitude they have, and in every body movement they use to express emotion, etc. etc.
This might sound very demanding.
In a way, it is. But in another way, it's not so bad, because the pieces of a culture flow into one another. Usually there's an overarching world concept involved, an underlying principle, or a set of underlying principles. Even just a large metaphor, such as the metaphor of the hunt and the food chain that I used to structure the world of the Aurrel in "Cold Words." If you can come up with principles, then you can start to push deeper with your expressions of culture in a way that will make sense and that readers will be able to grasp. The important part is that the practices you create must make sense to the characters. They must appear logical and obvious - and if they are strenuous, then there must be a strong motivation for engaging in such strenuous activity.
If you can build culture into the actions, speech, and thoughts of your character, then you won't have to explain, or work hard to have some character in your story explain how the culture works.
It's something to think about.