Actually, in the original German, Marx used the words Socialism and Communism interchangeably in many of his writings. (Kommunismus and Sozialismus) It was Vladimir Ilyich Lenin who first differentiated between the two and started using socialism to refer to a transitional state between "lower" and "upper" Communism.

Of course, the socialism that most people refer to doesn't correlate with any correct definition (more closely "democratic socialism"), but what do we expect from the "unwashed masses". ;)