Speaking strictly from a political point of view, "Liberal" can mean one of three things, depending on where you are in the world.
In Europe, and other democracies other than the United States, "Liberal" often refers to a person or group that favors little government intervention in the economy, low taxes, strong personal liberties, and a generally secular society, but usually only to the point of being ambivalent to religion. They often fall in the center of the political spectrum with Social Democrats and Socialists to the left (who favor strong government regulation of the economy, high taxes, and secularism to the point of being hostile to religion) and Conservatives and Christian Democrats to the right (whose political positions vary greatly depending on the country). A European "Liberal" can be equated with a "Libertarian" in the United States.
In the United States, "Liberal" basically means the same thing that "Social Democrat" or "Socialist" means in Europe. They are opposed, on a large scale, only by American-style Conservatism.
In the rest of the world, "Liberal" usually refers to a person or group that is pro-democracy and (especially in the Middle East) pro-secularist.