This is old, but it's a damn good read: "What is Progressive?" by Andrew Garib, Campus Progress, posted in July at AlterNet. Clip:
The first key to understanding progressivism is that it's not the same as liberalism, as many might assume. "Progressivism is an orientation towards politics," Halpin said in an interview with Campus Progress. "It's not a long-standing ideology like liberalism, but an historically-grounded concept ... that accepts the world as dynamic." Progressivism is not an ideology at all, but an attitude towards the world of politics that is far less black-and-white than conservatism or liberalism, breaking free from the false and divisive dichotomy of liberal vs. conservative that has dominated American politics for too long.
Said simply (perhaps oversimplifying), American liberalism is an ideology grounded in traditionally liberal American values: individual freedom, democratic government, freedom of thought and belief, and equal opportunity. Government intervention is generally seen as the solution to society's problem.
Progressivism, on the other hand, is far more flexible than any one ideology. Traditionally, conservatives see the world, especially human nature, as predictable and static. Liberals are often burdened with endless optimism - a belief that all problems can be solved through implementing utopian visions (especially through government intervention).
Progressives aren't simply liberals; progressives see the world for what it is, accept it as ever-changing and dynamic, and choose the best course of action in line with decidedly American values.