Its a system of government where everyone shares all the wealth equally. As nice as that sounds the leaders are always corrupt, so you end up with rich dictators, poor defenseless peasants, and a worldwide view of it as evil when in pure form its actually not a bad idea.
Basically the belief that all system of social organization based on the holding of all property in common ownership and basically being all part to the community as a whole or to the state. So everyone has equal property given by the government.
Communism is revolutionary socialist movement to crate a classless, moneyless and stateless social order. Its starts by Maxist-Leninist interpretation. Communist countries are China, Vietnam, North Korea and Cuba.