Syndicalism is the wing of the labor movement that refused the state entirely, holding that the union is not a pressure group inside capitalism but the germ of the society that replaces it.
Its argument is against both halves of orthodox socialism. Parliamentary parties, it held, are captured by the institutions they enter, and reform bargained through a legislature buys off exactly the militancy that could have won something; but a revolutionary party seizing state power only substitutes a new set of masters for the old. The alternative is direct action by the workers in their own industries, culminating in the general strike, after which production is run by the federated unions themselves.
Georges Sorel gave it a philosophy in Reflections on Violence (1908), and a strange one. He argued that the general strike works as a myth — an image that organizes will and moral energy — and that its value does not depend on its ever arriving. Coupled with his contempt for parliamentary compromise and his admiration for heroic struggle, this made him readable in more than one direction; syndicalists and, later, fascists both claimed him, and Mussolini had been a revolutionary syndicalist before he was anything else.
In practice it was the dominant current in French, Spanish and Italian labor before 1914, in the Industrial Workers of the World in America, and in the Spanish CNT, which in 1936 briefly ran much of Catalonia on its principles. It was broken between the communist parties on one side and the state on the other, but the argument it made — that emancipation cannot be delivered to people by an apparatus acting on their behalf — survived it.