What is culture?
Wow... this is a huge question. As you can see from the link that I am providing, there are a lot of ways that you can define culture.
To me, culture is the sum of all the things that most people in a society believe and do.
When you think about it, that covers everything, right? It includes how much and what kind of clothing we wear. But it also covers what we eat and what sporting events we watch and how we behave when we do those things. It includes what sorts of religious values we hold. And it goes down so far as to what sort of beds we sleep on. In other words, it includes everything.
Culture, then, is pretty much everything about a society -- everything that is common to the majority of the people in a society.
I would add that a good way to define "culture" is in opposition to "nature." That's a hugely widespread and influential way of defining the term. "Culture," in other words, is everything that "nature" is not: it's not what we're born with, what we're genetically programmed to do, etc. (In this opposition, you'll often see people use the word "nurture" in place of "culture.")
The hard part, of course, is sorting out exactly what is and what is not "natural." Noone grows up in a vacuum, making it nearly impossible to tell what's behind all of our thoughts and actions.
The opposition itself is controversial in many circles. See the links below for some examples.