In American politics, "progressive" is a euphemistic term that means more aggressively Leftist than a mere "liberal". Ironically, they began using the word "liberal" a century ago as a cover when the word "progressive" became unpopular. Today, the term "progressive" is used to show the specific area that they want to focus on. Progressive policies, in political science, are those which make progress towards goals seen by progressives as benefiting society. Since all politicians claim that their ideas and policies are meant to benefit the public, calling a policy "progressive" may be thought of as meaningless.
Historically, progressives used their term to describe themselves and their policies in contrast to "regressive" policies of their opponents. For example, progressives will ask rhetorically, "Do you want to go back to the 1950s?" implying that women were oppressed by being forced to be housewives and men were oppressed by being falsely accused of favoring Communism (see McCarthyism).
The term progressive was recently revived by politicians who considered themselves to be liberal. This revival occurred shortly before the 2006 elections. It was implemented to help distance politicians, mainly Democrats, from the negative history associated with various members of the party at that time. It was also meant to create a belief that conservatives were not able to think of a progressive future for the United States. The most famous historical usage of the term was in the 1890s to 1920s, sometimes called the Progressive Era. During the Progressive Era, politicians of both parties and various ideologies adopted the term, notably Theodore Roosevelt, a Republican who founded the Progressive Party (also known as the Bull Moose Party) and Woodrow Wilson.
The term has been popular among the media. And although most of the population still refers to the two major philosophies as conservative and liberal, it is often used to describe a more specific type of liberalism. This is likely because these two titles are familiar to the current population and better recognized. Because of this familiarity, it is easier to visualize the contrasts between conservative and liberal than the contrasts between conservative and progressive, because of the specific nature of "progressive".
A useful distinction to keep in mind is this: "progressives want bigger and supposedly better government; conservatives want less and supposedly better government."