Short answers, pulled from the story.
Social democracy is a broad, centre-left social, economic, and political ideology within the wider socialist movement. It supports political and economic democracy and a gradualist, reformist, democratic approach toward social equality. In modern practice it combines a predominantly capitalist economy with a robust welfare state.
Modern social democracies retain a capitalist system, while democratic socialists continue to advocate for its abolition. The philosopher David Schweickart defines social democracy as an attempt to strengthen the welfare state and democratic socialism as an alternative economic system to capitalism. Democratic socialism can also include revolutionary means, whereas social democracy accepts only representative democracy under the rule of law.
August Bebel, Eduard Bernstein, Friedrich Engels, Wilhelm Liebknecht, Karl Marx, and Carl Wilhelm Tolcke are all considered founders of social democracy in Germany. Bernstein rejected orthodox Marxism's revolutionary foundations and argued socialism should be achieved through gradual legislative reform, an approach he called evolutionary socialism. He described Marx as the father of social democracy while insisting Marx's thought needed revision.
The Third Way is an offshoot of social democracy developed in the 1990s that fuses economic liberalism with social democratic economic policies and centre-left social policies. It was embraced most notably by Bill Clinton's New Democrats and Tony Blair's New Labour. Some political scientists characterize the Third Way as effectively a neoliberal movement.
After the October Revolution, those who supported it renamed themselves Communists while those who opposed the Bolsheviks kept the Social Democrat label. The Communists wanted distance from social democracy's association with reformism. The division culminated in the German Revolution of 1919, where Communists sought a soviet republic and Social Democrats preserved the Weimar Republic.
The United States never consolidated a major socialist party, with the weakness linked to the Red Scare and the social stigma attached to socialism. A major contributing factor was the legacy of slavery, which institutionalized racial divisions within the working class and produced a segmented labour market. This fragmentation undermined class solidarity and constrained mass support for redistributive policies.
The United Nations World Happiness Report shows the happiest nations are concentrated in social democratic countries, especially in Northern Europe where the Nordic model is applied. The Nordic countries also rank highest on real GDP per capita, economic equality, public health, life expectancy, and human development. Countries practising a neoliberal form of government have registered relatively poorer results.