If you really want to know (and not just run down "liberals") this is a good start:
http://en.wikipedia.org/wiki/Liberalism
Liberalism refers to a broad array of related ideas and theories of government that consider individual liberty to be the most important political goal.[1] Liberalism has its roots in the Western Age of Enlightenment.
Broadly speaking, liberalism emphasizes individual rights and equality of opportunity. Different forms of liberalism may propose very different policies, but they are generally united by their support for a number of principles, including extensive freedom of thought and speech, limitations on the power of governments, the rule of law, the free exchange of ideas, a market or mixed economy, and a transparent system of government.[2] All liberals – as well as some adherents of other political ideologies – support the form of government known as liberal democracy, with open and fair elections, where all citizens have equal rights by law.[3]
Liberalism rejected many foundational assumptions that dominated most earlier theories of government, such as the Divine Right of Kings, hereditary status, and established religion. Social progressivism, the belief that traditions do not carry any inherent value and social practices ought to be continuously adjusted for the greater benefit of humanity, is a common component of liberal ideology. Liberalism is also strongly associated with the belief that human society should be organized in accordance with certain unchangeable and inviolable rights. Different schools of liberalism are based on different conceptions of human rights, but there are some rights that all liberals support, including rights to life, liberty, and property.
Within liberalism, there are two major currents of thought that often compete over the use of the term "liberal" and have been known to clash on many issues. Classical liberals emphasize free private enterprise, individual property rights, laissez-faire economic policy, and freedom of contract, and oppose the welfare state. Classical liberals support equality before the law, but hold that economic inequality, arising from competition in the free market, does not justify wealth redistribution by governments. [4] Social liberals advocate a greater degree of government influence to protect individual rights (in a broad sense), often in the form of anti-discrimination laws. Social liberals support universal education, and many also support the provision of welfare, including benefits for the unemployed, housing for the homeless, and medical care for the sick, all supported by progressive taxation.