Karl Marx

"Historical materialism" relates to Marx's belief that everything in society is determined by the things that we produce in order to live. In this theory of history, which is a theory for why things happened the way they did, Marx argues that society has progressed as a result of its material conditions (the things we have produced and the ways we have produced them) rather than a result of ideas and human thought.

Put simply: The most determining factor in society is the goods that we produce and the methods that we use to produce them.

Marx believed that society is divided into the base and the superstructure. The base is comprised of the material aspects of society and it allows for the production of goods. This includes the means of production (factories and tools, for example) and the relations of production (the roles performed by those involved in the production of goods). The superstructure, on the other hand, consists of all the other aspects of society, including culture, ideology, social institutions, political systems, and the identities that we subscribe to. According to Marx's beliefs, the superstructure is shaped by the base.  In turn, the superstructure legitimizes the base by portraying what might be unjust and exploitative as natural.
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Put simply: Society is built upon the ways in which we produce things, and this base is maintained through superstructures that legitimize those processes. 

Marx believed that the ways that society functions are dictated by a small wealthy class of people, the bourgeoisie, that own the means of production, such as factories and resources. At the opposite end of the spectrum, exists the proletariat who sell their labour to the bourgeoisie in order to survive. Since the bourgeoisie sells their products back to the proletariat for more money than they pay them for their labour, there is always an uneven distribution of power and wealth in society. As a result, there is always a conflict in society because the bourgeoisie strives to maintain their financial and political advantage over the majority working class.

Put simply: An uneven distribution in power and wealth causes conflict in society because the minority upper classes work to maintain their advantage over the minority lower classes. 

One way that the bourgeoisie maintains its power over the proletariat is by instilling within them a worldview that suggests that the system they live within is natural and justified. This is done through their control over the superstructure such as its confutative parts, such as politics, the media and education. False consciousness, according to Marx, is the inability to understand the oppressive nature of this class system. Class consciousness, on the other hand, is gaining awareness of this class system, their role within it, and how they contribute to its maintenance. Marx theorized that class consciousness would inevitably result in a worker-led revolution wherein the proletariat would overthrow the bourgeoisie and take back the means of production.

Put simply: False consciousness is the belief that the oppressive class system that exists in society is both natural and justified. Class consciousness is gaining awareness of this system and the role that we play within it. 

In the pre-industrial age, a worker would have complete control over the things that they produced. However, after the industrial revolution, labour became divided and people were reduced performing repetitive roles within a system. Consequently, through the wage-labour agreement, working-class people became alienated from the products that they produced as these products were designed. Additionally, work is no longer rewarding, the proletariat becomes objectified as replaceable parts of production, and they become disconnected from other workers because of competition.  

Put simply: Because of the role the proletariat play in a class system, they become detached from their work, their sense of self, and their community.