One of the most misunderstood theoretical concepts in Marxian social thought. And frustratingly so. In broad terms base/superstructure refers to a relationship of ontological presupposition. Or to put it in plainer English, "What has to exist in order for X to exist? Before X can exist?" Water, for instance, ontologically presupposes hydrogen molecules. But two phenomena that ontologically presuppose each other need not be empirically distinct phenomena. So the "superstructure" of water, as an example, actually contains the "base" of hydrogen molecules. What this means is that base/superstructure does not map onto the binary of material/ideal (e.g., scientific knowledge is very much a "basic" force of production), nor does it refer to a mechanisms for grouping distinct institutions. Per the example above, is water a part of the "superstructure"? Or part of the "base"? Or both? What about the family? Doesn't it contain the very basic function of (re)producing labor power? You can see that once the concept of base/superstructure is really grasped, these categorization questions don't make much sense. The state, in one sense, is superstructural -- in that it results from and in turns coercively shores up power that is derived from the "base" (control of productive resources) -- but in another sense, especially as the state takes on more of a role in capital accumulation with the rise of monopoly capitalism, very much performs "basic" functions.
The oft-quoted remark from the 1859 Preface quoted above happens to be making the point that I allude to above about the origins of political power lying in control over productive resources.
^ No disrespect to the writer but that's an unnecessarily obfuscated explanation (for the learning forum).
How about this way of looking at it?
A ship has a hull and a superstructure on top of that hull. Water is the equivalent of the ship in it's entirety, combing two elements to produce a more complex chemical compound.
Society has two important 'elements' to it. Civil society (productive forces and social relations) as the economic base and political society (the political system and institutions combined which are linked to it) as the political superstructure.
The economic base isn't just something as simple as the working class though, it is a combination of all things directly related to it such from a worker's mental state to how the means of production are organised.
The two are sometimes considered to be distinct from each other but also interrelated depending on your viewpoint. I prefer to say that the two are intertwined and cannot be separated, only modified or overhauled/revolutionised.
"Quotations are useful in periods of ignorance or obscurantist beliefs."
- Guy Debord (Panegyric)
"Guided by the Marxist leader-dogmas of misbehaviourism and hysterical materialism, inevitably the masses will embrace, not only Groucho Marxism, but also each other."
- Bob Black (Theses on Groucho Marxism)
"I think that the task of philosophy is not to provide answers, but to show how the way we perceive a problem can be itself part of a problem."
- Slavoj Žižek ("Year of Distraction" lecture)