In US politics, progressive is sometimes used as a synonym for "liberal", used to evade the bad connotation of the latter term. Progressive policies, in political science, are those which make progress towards goals seen as benefiting society. Since all politicians claim that their ideas and policies are meant to benefit the public, calling a policy "progressive" is thus practically meaningless.
Liberals use the term to describe themselves and their policies in contrast to "regressive" policies of their opponents. For example, they'll 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, for 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.
While the term has been popular among the media, most of the population still refers to the two major philosophies as conservative and liberal. 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.
A useful distinction to keep in mind is this: "progressives want bigger and supposedly better government; conservatives want less government."