In Marxist thought, the base of a society consists of its means of production, material assets, and the classes of people who produce and consume things (described by Marxists as the "relations of production" and divided into Lumpen-Proletariat, Proletariat, Labor Aristocracy, Petty-Bourgeoisie and Bourgeoisie).

The superstructure is the ideology of a society. It includes the laws, religion, educational system, art, philosophy, politics, social customs and traditions, the news media, literature and everything else that is influential in a society that is not directly concerned with production.

In The German Ideology, Marx and Engels reject the Hegelian theory that the progress of history depends on changes in ideology. Instead, they argue, real change is brought about by shifts in economic and material relations in the base, which periodically bring new classes to power. The superstructure then changes to justify the power of this ruling class. The base therefore shapes the superstructure, though the superstructure then stabilizes the base by providing it with ideological legitimacy in the eyes of the masses.

In Marxist thought, the base is the economic foundation of society. It comprises of the forces and relations of production, the division of labor, property relations, and so on. The superstructure, which arises from the base, consists of such things as political institutions, culture, the legal system, and society's governing ideology.

Although the superstructure is ultimately derived from the base, it's not necessarily the case that the casual relationship between the two goes in one direction. For instance, Marx acknowledged that laws—part of the superstructure—could determine property relations between the bourgeoisie and the proletariat, which are part of the base. In fact, he recognizes that there couldn't be economic relations between the classes of any kind without a prior legal framework in place. That being the case, it's best to understand base and superstructure in Marxism as conditioning each other in a complex relationship that develops over time as society itself develops.

Basically speaking, Marx says that the "base" is the foundation of our society.  It is the thing that is truly important.  By contrast, the superstructure consists of things that are built upon that base.

Marx argued that what was truly important in any society was the "relations of production."  In other words, it is the relationship between those who do the work and those who own the means of production that really determines how a society is.

On top of that base, there is a superstructure that seems to be what is important about our society.  This consists of things like our structure of government or our culture.  These are things that seem really important but aren't.  What is really important is what the relationship is between those who work and those who own the means of production.