Communism is an ideology that everyone is equal - noone is rich, noone is poor. Invented by Karl Marx, a German living in England who saw the oppression of the working class workers who worked long hours with very low wages while the rich bosses (capitalists) were living a luxury life. According to the theory of communism, killing was natural meaning that a bloody revolution was necessary to overthrow the capitalists and other anti-communists. In the end, society will be left classless with everyone agreeing with Communism so therefore, everyone is equal and shares the same beliefs. Of course, it's easier said than done. Religion is banned and Darwinism is a belief used by Communist which is Darwin's theory of evolution. The reason why religion is banned is because every religion has different beliefs which causes conflict with other religions. By believing in Darwinism and not religion, the theory of equality remains with everyone sharing the same belefs.
In capitalist countries (democracy), private ownership is allowed and there are alot of wage differences with the consequence of wealth gap. In communism, everything is owned by the state and they fully control the economy (like deciding what is produced and how much). This means full employment of labor at the cost of lower standards of living due to wages lower than the capitalist countries.
Another theory of communism was that when countries become communist, they will never fight each other because both countries share the same beliefs of equality. This theory failed as communist countries had seperated or fought each other like the Vietnamese invasion of Cambodia to take out the Khmer Rouge or China's seperation from the USSR.
Communism has left many unanswered questions like for example, in terms of equality does it mean that a doctor earns the same wage as a nurse?
Today there are only five communist countries - China, Vietnam, Cuba, Laos and North Korea.