Social democracy is a strain of liberalism that, in its current meaning, seeks to merely modify parameters within which capital operates with the aim of "humanising" or "taming" capitalism. Social democracy pursues the increasing of welfare of working people through social programs. However, social democracy has gone through multiple meanings, the oldest of which was revolutionary.
First Phase: Marxism (circa 1875-1910)
In this phase of social democracy it was a Marxist movement advocating the revolutionary overthrow of private ownership in favour of social ownership. It was associated with the Second International that existed between 1889 until 1916. The majority of social-democratic parties backed their national governments when they entered into World War I, an event sometimes referred to as the Great Betrayal.
Second Phase: Revisionism (circa 1910-1945)
Revisionism is associated with Eduard Bernstein.
Bernstein disavowed scientific socialism in favour of ethical socialism.
Third Phase: From Consensus to Crisis (circa 1945-1970s)
After World War II a consensus developed around social-democracy and Keynesianism. Paternalistic conservative and social-democratic parties built welfare states and nationalised key industries such as telecommunications and public transportation. This period coincided with the post-World War II economic boom.
In this period, the aim of social-democracy was the elimination of poverty, cradle-to-grave welfare provisions, creating full employment, and it upheld values such as class politics and the nation-state.
In the 1970s this period ended when social-democracy and Keynesianism hit a crisis. The 1973-1977 recession wp was the primary catalyst for the transition to neo-liberalism in the 1970s. This was compounded by the fact that inflation and unemployment rose simultaneously, whereas Keynesians had theoretically 'proved' that there was a negative trade off between the two phenomena.
Fourth Phase: Neo-revisionism (circa 1970s-present)
Globalisation had forced social democracy into crisis and it had to reinvent itself. This produced what is referred to as neo-revisionism and is related to neo-liberalism and the Third Way championed by Clinton, Tony Blair, and Dutch former prime minister Wim Kok.
Socialism, meaning state intervention, was declared dead and class conflict was replaced by a consensus view of society. Key is the concept of a market state, based on social investment in infrastructure and people geared toward global competitiveness. Thus, education warrants state investment to create a capable work force for the particular needs of capital of the information age, rather than to enable self-actualisation and cultural enrichment. Many social-democratic parties have, on this basis, supported austerity measures in the aftermath of the 2009-2014 economic crisis.
Full employability has replaced the goal of full employment, and the aim of cradle-to-grave welfare provisions has been replaced by the notion of the concept of welfare-to-work. It upholds values such as meritocracy, equality of opportunity, and globalisation.
Democratic socialism developed as anti-neoliberal reaction to neo-liberalised social democracy. This current is similarly committed to the aims of the third phase of social democracy, namely full employment, social welfare, nationalisation of key industries, and opposition to deregulation and liberalisation.
Chavismo is considered a form of democratic socialism. 
Eurocommunism is a reformist deviation from Marxism-Leninism.
Various Marxist-Leninist parties have transformed into Eurocommunist parties, including the Communist Party of Spain, the Japanese Communist Party, and the now defunct Communist Party of the Netherlands and the Mexican Communist Party. Presently, Synaspismós (SYN), which dominates the party SYRIZA [wp], is the largest Eurocommunist party globally. SYRIZA has achieved major electoral success and was backed by famous Marxist Slavoj Zizek. It has moderated its stances since 2014. The party's leader Tsipras "promises Greek businesses tax breaks and reforms that can bolster healthy enterprise and leave back the parasitic, state-funded business model of the past" 
- 4th August 1914: The Great Betrayal and Collapse of the Second International. Accessed December 9. http://www.marxist.com/4th-august-1914-the-great-betrayal-and-collapse-of-the-second-international.htm
- Andrew Heywood. Political Ideologies. 3rd edition.
- Kerin Hope. December 10, 2014 6:00 pm. Greece’s radical left Syriza seeks to soften its sharp edges. Accessed December 11. http://www.ft.com/cms/s/0/577efe3a-8080-11e4-9907-00144feabdc0.html
- Robert Stevens. 2014. Greece’s SYRIZA steps up its overtures to big business. Accessed December 11. http://www.wsws.org/en/articles/2014/07/04/gree-j04.html
- George Gilson. 2014 May 29. Tsipras builds bridges with business. Accessed December 11. http://www.enetenglish.gr/?i=news.en.article&id=1980