oh and here is another definition i just found that i think explains some other points:
Communism is a political ideology that seeks to establish a future, without social class, or formalized state structure, with social organization based upon common ownership of the means of production. It can be classified as a branch of the broader socialist movement. Communism also refers to a variety of political movements which claim the establishment of such a social organization as their ultimate goal.
Early forms of human social organization have been described as "primitive communism." However, communism as a political goal generally is a conjectured form of future social organization which has never been implemented. There is a considerable variety of views among self-identified communists, including Maoism, Trotskyism, council communism, Luxemburgism, and various currents of left communism, which are generally the more widespread varieties. However, various offshoots of the Soviet and Maoist forms of Marxism-Leninism comprise a particular branch of communism that has been the primary driving force for communism in world politics during most of the 20th Century.