Socialism is an economic system that requires public ownership and government control over the production of goods. Government is supposed to make decisions about what and how much to produce in an attempt to increase the common welfare. Socialism is common in most parts of the world outside of the United States.
Socialism is described by this saying: "From each according to his ability, to each according to his labor." Socialism allows private property in the form of wages to workers, but not in the form of ownership of large companies.
Karl Marx considered socialism to be a transitory strage between capitalism (full private property rights) and communism (almost no private property rights).