The Liberal definition is not easily understood. You can ask about the governmental dicotomy of Liberal versus Conservative idealogy and you'll get 100 different answers from 100 different people. The differences between Liberals and Conservatives may seem huge to the average American citizen, but once you begin to explore, you'll see certain elements of Conservatist thought that rears itself in Liberal ideas. The truth is, in a perfect world, you'd be able to give a concrete defintion to Liberals. But, this world isn't perfect, so nor is the definition for a Liberal.
The cut and dry definition of a Liberal is someone that believes in equal opportunity for all to advance and succeed. Liberals believe that it is the government that should police society and protect each and every one's civil liberties. The government should be a safety net of sorts, protecting all people. Liberals believe in making sure that no one is in need.
In order to understand Liberal idealogy, you must have a basic understanding of Conservative thought as well. Conservatives are for limited government and free markets. They believe that the individual should take more of a role in securing his own place in society. It's no wonder why so many rich folks in the country are Conservative and the poorer folks tend to side with Liberal thought. It's much easier to believe in pulling yourself up by your bootstraps when you can afford said bootstraps. Just as it's easier to understand why some people need government to help them out.
While those two definitions describe Liberals and Conservatives in a cut and dry fashion, you all know that nothing is ever cut and dry. You need to look at the dicotomy of Liberalism and Conservatism as a range instead of two separate entities. People fall somewhere within that range. Some Liberals lean a little closer to Conservative ideas, just as some Conservative folks may have liberal ideas for certain facets of life. It all comes down to an individual personal politics and the lesser of two evils in his mind.