Communism is an ideology and a system of government. The philosophical basis for communism springs largely from the 19th-century works of Karl Marx and Friedrich Engels, who wrote out of concern for the suffering of working people under the extreme conditions of early industrialization. This philosophy eventually influenced the governments of entire nations. The Soviet Union was the most prominent communist country in the 20th century. The People’s Republic of China is the most prominent communist country today.
The basic idea behind communism is that under capitalism there are two types of people: the few (bourgeois) who own the means of making money, and the masses (proletariat) who work to support the few. The only way to change this, according to communists, is to overthrow the bourgeois. When this is done, then everything can be owned in common for the public good.
Communists and socialists share a concern for working people, but have many differences. Socialists believe in democracy and private property. Communists believe that democracy is a sham and that (in theory) no one should be allowed to own property that they do not directly make use of. Historically, communists have advocated violent revolution and socialists have advocated a political revolution in attitudes about how society treats the working class.
Practically, communist societies can be very different: for example, communist China today is organized around a powerful economy driven by businesses, while communist North Korea is a primitive family dictatorship.