On May 5th, 1818, Karl Marx, hero of the international proletatiat, was born. His revolution of Socialist theory reverberates throughout the world carries on to this day, in increasing magnitude. Every passing day, he is vindicated. His analysis of Capitalism, development of the theory of Scientific Socialism, and advancements on dialectics to become Dialectical Materialism, have all played a key role in the past century, and have remained ever-more relevant throughout.
He didn't always rock his famous beard, when he was younger he was clean shaven!

Some significant works:
Economic & Philosophic Manuscripts of 1844
The Eighteenth Brumaire of Louis Bonaparte
The Civil War in France
Wage Labor & Capital
Wages, Price, and Profit
Critique of the Gotha Programme
Manifesto of the Communist Party (along with Engels)
The Poverty of Philosophy
And, of course, Capital Vol I-III
Interested in Marxism-Leninism, but don't know where to start? Check out my "Read Theory, Darn it!" introductory reading list!
Isn't capitalism itself not only like 100-150 years old?
It really arose during the Industrial Revolution, around 1760. Imperialism, the final stage of Capitalism, began towards the end of the 19th century.
So how is 19th century imperialism different from Roman imperial expansion or Greek colonialism in antiquity for example? Or the various attempts to resurrect the Roman Empire by basically everyone? Why don't we call Roman emperors or Alexander the Great or even Sigismund imperialists? Honest question here, I'm not a historian or anything.
Imperialism can occur in any class society. In its most general definition, it means the theft of land, labor, and resources of a weaker country to feed a stronger one. So we do call it "Roman imperialism":
The surplus here is an agricultural one.
Imperialism takes a different shape under capitalism, where instead of a landed aristocracy / slave-owning class doing the colonizing, its finance capitalists in the imperial core exporting production to low-wage / underdeveloped countries to produce commodities cheaply.