Two Concepts of Liberty was a lecture, and subsequently a pamphlet, by the philosopher Isaiah Berlin in 1958. It is famous for contrasting the ideas of negative and positive liberty (the terms negative and positive are not meant as judgments of the two ideas, though Berlin himself came out strongly against positive liberty).
Negative liberty is the freedom from interference or restraint, exemplified by minimal government, property rights, free speech, freedom of worship, free movement. It is called negative because it exists through the absence of coercion or tyranny.
Positive liberty is harder to define simply, it refers to the ability of an individual to fulfil his or her potential through means other than self-reliance. It is exemplified by things like affirmative action, redistributive taxation, government work programs and other socialist policies.
In Two Concepts of Liberty Isaiah Berlin lectured that the idea of positive liberty has been used by governments to cover up abuse, leading to at first to paternalism, then the curtailment of people's freedom "for their own good", and finally to communism.