At its core, socialism is an ideology that encompasses a variety of economic and social systems that are defined by communal ownership of output rather than individual ownership.
At a glance, socialism has philosophical origins at least as far back as ancient Greece. Plato had envisioned a common society in his work Republic (360 B.C.).
Socialism gained traction as a response to the ever growing capitalist market. This is especially true as the Industrial Revolution brought on a greater gap in equality among laborers.
Marx was one of the most noteworthy supporters of socialism. In his work The Communist Manifesto he explained that society is divided into classes where certain classes control the means of production.