First, "liberals" do not all believe exactly the same stuff. Nor do conservatives for that matter.
From an online political dictionary:
liberalism - in the nineteenth century in Europe, the great age of liberalism, the term stood for freedom from church and state authority and the reduction of the power of royalty and aristocracy, free enterprise economics, and the free development of the individual. Liberalism advocated freedom of the press, religious toleration, self-determination for nations. It was liberalism that established parliamentary democracy. The Founding Fathers might be termed liberals. In the twentieth century, liberal parties were caught in between conservatives and socialists and their influence declined. Today, liberalism stands for something rather different than it did in the nineteenth century (more government rather than less government). See also liberal.
liberal - in political speech now in the U.S. a liberal is a person who believes it is the duty of government to ameliorate social conditions and create a more equitable society. Liberals favor generous spending on the welfare state; they exhibit a concern for minorities, the poor, and the disadvantaged and often see these conditions as a product of social injustices rather than individual failings. This also applies to crime and juvenile delinquency, where liberals are as concerned with removing the social causes of such behavior as they are with detection and punishment. Liberals also tend to be concerned about environmental issues, the defense of civil liberties, and do not favor excessive military spending. The label of liberal is something that many politicans now seek to avoid, since it is out of keeping with the public mood. In the presidential campaign of 1988 George Bush used this to telling advantage, labeling his Democratic opponent Michael Dukakis a liberal, and making the term sound subversive and un-American. President Clinton tried to distance himself from traditional liberalism in his campaign of 1992, calling himself a New Democrat instead. See also liberalism.
[side note: In Canada, our most centrist party are the Liberals, but our most leftist party is the New Democrat Party. Go figure.]