Socialism is an economic and social system defined by social ownership of the means of production. (Workers democratically own and operate the places in which they work, as opposed to private power aka capitalism)
The means of production are non-human inputs the create economic value, such as factories, workplaces, industrial machinery, etc. Socialists refer to the means of production as capital, or private property. Private property in the socialist context shouldn't be confused with personal property, such as your home, car, computer, and other possessions.
In a capitalist society the means of production are owned and controlled privately, by those that can afford them (the capitalist aka those with capital). Production is carried out to benefit the capitalist (production for profit). Workers are paid a wage, and receive that amount regardless of how much value they produce.
Communism is the highest developed stage of socialism wherein there is no state, no money, no class system. The means of production are owned by all and provide for everyone's needs.
Past and present socialist/anarchist societies include - Revolutionary Catalonia, Anarchist Aragon, Shinmin Province in Korea/Manchuria, Free Territory of Ukraine, The Bavarian Soviet Republic, The Paris Commune, The Zapatista controlled areas of Chiapas (current day), Magonista Baja California, Shanghai People's Commune, Rojava (current day), etc
10 minute intro to Karl Marx --- (Reminder for newcomers that private property refers exclusively to the means of production, not your home and other possessions which are considered personal property)
Introduction to Marxism by Professor Richard D. Wolff
Against Capitalism by Jerry Cohen
Introduction to Anarchism by Noam Chomsky
Here is a list of some more Chomsky videos
'Anarchy Works' - A simple Q&A style book
Albert Einstein - Why Socialism?
The Conquest of Bread by Kroptokin - Anarcho-Communism, audiobook
Socialism: Utopian and Scientific by Engels
Reform or Revolution by Rosa Luxemborg
Anarchism and Other Essays by Emma Goldman
Capital Volume 1 by Karl Marx - Explained by David Harvey
Mutual Aid: A Factor of Evolution
The Zapatista movement: A modern day libertarian socialist society
Rojava: Another modern day socialist society
Marx's concept of false consciousness; similar to what we call 'the matrix' in the 21st century
Marx on 'alienation' of workers
Liberationschool.org
/r/Anarchy101 <--- the best of all the '101' subreddits IMO as it has the most content (use the search bar as well)
https://youtu.be/-w12bkm9g8o?t=3m18s <--- Capitalist exploitation explained
Leftist movies and documentaries:
'Inside Job' - documentary featuring Matt Damon about the 2008 financial crisis
'The Perverts Guide to Ideology' - In this clip from the film, Slavoj Zizek explains ideology