Monism and pluralism both are examples of materialism. Materialism is a kind of metaphysical theory that asserts that reality, the universe, is made of some kind of material stuff. Monism is a term that describes any philosophy that denies that they are many separate beings in the universe. It is also known as a metaphysical theory that attempts to reduce all variety if entities and things down to just one type of matter or thing. Pluralism on the other hand is a metaphysical theory that states that there are a multitude of basic components that make up the universe. These two theories may sound similar, but it is very different from each other.
Monism states that if you analyze the universe at its deepest level, it is only composed of one fundamental kind of stuff. There are many types of monism. Material monism is a main one. Before the inventions of technology we have today, most philosophers thought that this specific monism states that there is but one reality, matter, whether it be an agglomerate of atoms, a primitive, world-forming substance, or the so-called cosmic nebula out of which the world evolved. Pluralism is a theory that states that ultimately, there are many kinds of substance, not just one. Pluralism is a way of saying that the universe has unity, but not the unity of only one thing, but many things that have the fact of existence in common. Aristotle and Aquinas explained the pluralism of beings by composition. Composition being that is presence of two elements coming together in one thing such as form or matter.
Since the beginning of monism, many philosophers have put their theories out there. For example, Thales theorized that all matter was composed of water. And Heraclitus theorized that fire was the basic underlying element. Since then, a debate started: was the world really made of water or fire? This debate settled down after Democrats posited the existence of atoms.