There are hundreds of perfectly respectable definitions of the term "culture", and there is no shortage of different modes of usage of the term. But what connects most useful definitions is a simple principle: culture is that which creates meaning in human interactions, which makes human behavior meaningful and comprehensible in the eyes of others.
This means that:
-Culture is necessarily shared, it is - hold on to your chair now - a social phenomenon.
-It is also concrete, not abstract. When you in tend to observe culture, or do research on it, you will find very little usage in sources in which culture is defined as a metaphysical set of historically shared traditions, values or what not. Culture is what people make of it.
-It is normative, in the sense that human conduct can only be recognizable and comprehensible to others when it matches established or known ways of performing such conduct. Saying that it is normative also means that it is subjective and permanently submitted to evaluative (moral) judgments by others.
-In spite of its sharedness, culture is never unified or homogeneous. It is fragmented and enacted in very different ways by different people. Young people, for instance, may have a different cultural idea of "school" than their parents. And this is normal.