Here is a helpful summary by Sean Michael Lucas (who blogs here) in his book On Being Presbyterian:
...three progressive intellectual trends became prominent at the PCUS seminaries. The first, and most important, was the "Social Gospel." The phrase described the effort to relate biblical principles to social needs and challenges raised by the industrialization and urbanization of the early twentieth century.
But the Social Gospel came to represent a major shift in the way important theological categories were used. In short, the Social Gospel represented a movement away from individual to corporate categories for theology. Sin was defined in social and systemic terms--the oppressive social structures that kept people from achieving their potential. Salvation, likewise, was the removal of those structures in order to maximize human potentialities and make a more just world.
Also distinctive about the Social Gospel movement was a genuine embrace of the historical Jesus and his teaching as the norm for social action; "What would Jesus do?" was the question that Social Gospel promoters...desired Christians to ask themselves. In particular, the question was what Jesus would do in order to realize the kingdom of God as an earthly reality, bringing social harmony in its wake.
All natural and political processes that brought God's kingdom to closer fulfillment were seen as the work of God's Spirit.
Sean Michael Lucas, On Being Presbyterian: Our beliefs, practices, and stories, p. 227-8