Short answers, pulled from the story.
The Social Democratic Workers' Party of Germany became the first social democratic party in Europe in 1869. This organization adopted socialism associated with Karl Marx as its official theoretical basis by that same year.
August Bebel, Wilhelm Liebknecht, Carl Wilhelm Tölcke, Eduard Bernstein, and Ferdinand Lassalle are considered founders of social democracy alongside each other. They argued socialism could be achieved through cooperation between people regardless of class rather than class conflict.
During the German Revolution of 1919 Communists wanted to overthrow the government and establish a soviet republic like Russia while Social Democrats wanted to preserve it as what came to known as Weimar Republic. The Bolsheviks split from Second International created separate Communist International Comintern in 1919 rally revolutionary social democrats together for socialist revolution.
Countries like Japan West Germany Austria South Korea France Italy Greece experienced unusually high sustained economic growth together full employment during this period. These policies were especially associated Swedish Social Democrats who implemented Keynesian economic policies industrial agreements balance power capital labour welfare state.
The reconceptualization social democracy developed 1990s embraced some social democratic parties political scientists analysts characterizing effectively neoliberal movement. Tony Blair New Labour Bill Clinton New Democrats notably embraced approach.