Karl Marx is one of the most influential and controversial thinkers of modern times. Though receiving some terrible PR from the (misguided) attempts to implement Marx's ideas, his philosophy is not only powerful and inspiring but also relevant to our times. Marx is typically portrayed as an economic thinker but he is actually a thinker of time, history and nature of human progress and his ideas touch on the very essence of human nature. His ideas about communism were not something to be implemented but rather something that has to eventually happen since history orders so. Marx's analysis of the way society function on the role terms such as class conflict, ideology and alienation within it are still relevant when we come to understand contemporary societies (largely due to the fact that capitalism, despite Marx's prophecy, is also still contemporary).
Marx's theory and ideas are extended and elaborated. Here are a few summaries to help you understand some of his most important texts, such as The German Ideology and the famous Communist Manifesto alongside some summaries that explain important ideas in Marx's theory.
Summaries of Marx's writings and ideas:
An Extended Summary of The Communist Manifesto by Marx and Engels (chapter 1, chapter 2, chapters 3 and 4)