Liberalism is often associated with other concepts as part of the foundation of Western government and social theory. Those concepts are:
Democracy, at its most basic, holds that people should have a say in their own governance. This doesn't tell us much, though. The ancient Greeks are said to have a democracy, but many people in that society did not have a say in their own governance. "Having a say", as well, may not mean having a direct say, but rather an indirect say, as in representitive democracy.
Sometimes also called "capitalism", market-based economics assumes that there should be as little intervention as possible from the government in production, consumption, and trade.
Finally, liberalism holds that our social and political life is governed by abstract and universal concepts. These could conceivably differ in different places, but usually the concepts include things like freedom/autonomy, equality, individualism, tolerance, etc.
Source for classical liberalism: Locke, Second Treatise on Government
Most people in the US use the term "liberal" in a way that is not the classical way. In the contemporary sense, "liberal" is contrasted to "conservative", and is the political doctrine that holds that "the purpose of the state, as an association of independent individuals, is to facilitate the projects (or "happiness") of its members." (Blackwell Dictionary of 20th Century Social Thought, 333)
It is worth noting that, although the three of these tend to come together as a package, they are not identical, and indeed, in some ways actually work against each other, if not in principle at least in reality.