Marxists Internet Archive
Encyclopedia of Marxism: Homepage
This site from the Marxists Internet Archive provides a detailed explanation including quotations from Engels and Marx, as well as explication. Links defining terms are helpful, as is the suggested further reading link.
The History Cat
The History Cat: Karl Marx: Father of Communism
Summarizes the capitalist conditions, brought on by the Industrial Revolution, that led to the fostering of the socialist ideas espoused by Karl Marx, and to the writing of The Communist Manifesto by Karl Marx and Frederick Engels.