-I am not gonna bother with biography, you can find those details in Wikipedia-
Main topic:
From all his great political and social innovations, Marx is most recognized for the change he brought on the way of perceiving world history. Up until then, historians and social scientists where based on the idea that the roots of every historical change were to be found in the constantly changing ideas of people and that the political changes where the most significant; those that really make history. But none of them was really trying to understand why people change their mentality and their way of seeing things. Only in the newest generation of some French and English historians, we see for the first time the idea that the driving force for all the great changes in European history was the constant battle of the classes, the battle for democracy and equality. Marx took this idea and upgraded it in a whole new level, by proving that every part of global history was in fact history of battles between the ?inferior? and the ruling classes, that all complex political struggles of the past, were indeed battles for the social and political dominance or for the preservation of the dominance of the ruling classes.
This new way of understanding history has the most important role in sociological studies. Being proven that every battle in history had a dominance of a specific class basis, sociologists of that time, were free from their anthropocentric frames and managed to conceive the idea of a global ?humanity? rather than just ?human societies of the past?.
The second greatest change Marx brought into the scientific community is that he finally explained the relation between political economy and labor (?Das Capital?), on other words he proved that every day in history is a day of the ruling class taking advantage of the people of the inferior class, as they are the ones who produce the good, yet they have limited access on them, as the main strings of the economy belong to those who possess a high political and financial status.
Furthermore he was the one who introduced the idea of ?historian materialism? or the "materialist conception of history" which is based on Hegel's claim that history occurs through a dialectic, or clash, of opposing forces. The fundamental proposition of historical materialism can be summed up in a sentence: ""it is not the consciousness of men that determines their existence, but, on the contrary, their social existence that determines their consciousness."

Preface to A Contribution to the Critique of Political Economy.), which means that above all our consciousness is determined by our being, by our current state of living at this specific moment which makes us what we are. Of course, Marx?s definition is not a dialectical one, he is strictly political. It is the social class in which we belong that determines the way we see and anticipate the world and the economical and political system.