Tyler Cowen and Arnold Kling attempt comically inept descriptions of progressivism through libertarian blinkers. But worse, neither of them see the big picture.
I'll take a stab at describing progressivism.
Progressivism is a political philosophy that takes the "pursuit of happiness" seriously. Pursuit of happiness in a more Aristotelian sense: human flourishing. Not beer parties. Flourishing means being able to become what you want and do what you want. Progressives want more human flourishing. In this sense, early liberals were progressives and most modern liberals are progressives. And not just flourishing for elites, but for everybody at every age.
Progressives see many obstacles to human flourishing: poverty, disease, tyranny, corruption, monopolists, bigotry, ignorance, traditions, pollution, crime, war, etc.
Progressives believe that humans can flourish better if these obstacles are removed or circumvented, and that society can find solutions. This improvement is the progress in progressive.
Some of the solutions to problems of human flourishing include markets, regulations, unions, corporations, education, laws, publicity, privatization, takings, antitrust, buyouts, public works, infrastructure, balancing powers, social provision, etc. Progressives are pragmatic: they are not committed to one set of solutions (such as traditional solutions or markets): they will look at the history of experimentation with these solutions accross the planet and select what they think will work best in their situation, even if it is untried. In this sense, the founders of the US and authors of its Constitution were progressive.
Progressivism demands improvement, but not perfection. Early liberal enfranchisement of white, male landowners with the vote was progressive, but far from perfect. Later enfranchisement of blacks and women was further progressive improvement. And civil rights voting acts to enable blacks and other minorities to actually register and vote was further progressive improvement still.
This doesn't mean that progressives all perceive the same problems and would choose the same solutions. Progressives are a heterogeneous lot, and can disagree strongly. But their pragmatism and lack of perfectionism allows them to work together and with others easily through compromise.
It may appear that progressives turn away from markets and towards government, but that is because there is little progress to be made by markets that markets aren't already making. In some cases, markets create problems (such as redlining) that require regulation to undo. In some cases, conspicuous market failures require either government incentives or government provision (such as for roads and schools.) In some cases, government institutions were under-performing (such as representation of the indigent in courts: the solution was access to free legal services.) And sometimes government is oppressive: hence the ACLU.
Progressivism has often been defined by its history and people. The history of progressivism is usually a list of issues that progressives have fought for such as 8 hour work days, universal enfranchisement, antitrust, etc. What unites all these issues is that they were viewed by progressives as solutions to the problems they saw for human flourishing. Lengthy work days provided no opportunity for leisurely pursuits, education, family matters, health issues, etc. Voting restricted to men meant women's issues (such as their legal status, whether they could own property, etc.) were not attended to. Monopolies and trusts created hardship for farmers and the poor by keeping prices artificially high.
Viewing progressivism by its history and people, rather than by its basic objective, causes a "can't see the forest for the trees" problem. But a moderately good list is available at Progressive Living.