The words 'socialism' and 'communism' are frequently used to describe the same kind of society - that is, an alternative to capitalism based on collective ownership rather than private property. In other cases, they are used to describe very different political ideologies - for example, the contrast between social democracy and Stalinism.
But for Marxists, the terms socialism and communism have very definite, scientific meanings. Marx himself used these words to describe the different stages in the transition from a revolutionary workers' state to a classless society.
In this latest video from the Marxist Student Federation's Myths of Marxism series, Thomas Soud discusses the relationship between these two terms and ideas.