Communism is a form of Socialism based on common ownership of business by the State (or Collective), with no social classes and no representative government. The Government (State) is authoritarian in nature and the people have no freedom of speech and very limited rights.
The State manages the economy and the population top-down, as it deems fitting. There is no private ownership of assets. Communism is considered to be the polar opposite of democracy because the government tells you what to do as opposed to citizens telling the government what to do. With Communism the government has the power versus the people having the power in democracy (as well as equality).