Karl Marx was a 19th Century economist who co-wrote the influential book "The Communist Manifesto"(thus creating communism) along with a fellow named Friedrich Engels, this book has become the root basis of all communistic theory since. Marx felt that it was historically inevitable that workers, who are paid only a small fraction of the value of what they produce, would rise up and overthrow the wealthy political leaders and company CEOs who were exploiting them.
Dictatorship of the Proletariat
Marx predicted the “Dictatorship of the Proletariat ”. This he imagined would be a temporary transitional system before true Communism emerged. The Dictatorship of the Proletariat is not rule by a Hitler or Stalin, but rather, the rule of a class. For this reason, it is actually, in the opinion of those who believe this, a form of Democracy. Unfortunately, the USSR didn't work out that way.
Whether or not Marx was correct, his economic and political theories have been very influential in both economic theory and as the inspiration for revolutionary movements throughout the World. Just as Adam Smith's view of a pure capitalistic free-market society has never existed, no society has ever managed to achieve the purely Communist system that Marx predicted.
Marx is one of the most hated and feared Bogeymen of your average Conservative, who suspects that there is a communist hiding under his bed waiting to grab his ankles. While some thought that conservatives might wake up once the Cold War ended and realize that there are more dangerous things to fear than communism (like a president who invades other Countries at random), they still resort to labeling anyone who is to the left of Attila the Hun a "communist" or "Marxist". Social Democrats and Democratic Socialists complain about being unfairly labelled as if they were revolutionary Communists. some people (& maybe 9/11 truthers) like Alex Jones & Mark Dice also feel the same way.