Marx's theories intersect both social and political aspects.
Of course, what we all hear about is the fact that he suggested the proletariat rise up against the capitalists.
In fact, he suggested such a situation would occur in part due to a crisis of overproduction. This would be the revolution towards communism which he felt was the ultimate social design which the world was moving towards as it progressed.
On a more social note, Marx had a lot of influence with respect to the study of changes within society due to the changing environment that people were working in.
This means that where we work and the conditions we work under has a lot to do with the fabric of our society. He felt that changing the factors of production, such as bringing people out of the agrarian lifestyle and away from individual artisanship towards a factory environment, allowed people to share a common experience.
Marx is generally summarily dismissed as being an ass for his beliefs with regard to communism, but his influence on social theory was significant. The study of how people interact in social settings and communism are two very seperate topics.