The political philosophy of non-initiation of force. This means that no one (not even the government) can force you to do something unless you violate someone elses rights by initative force against them.
Libertarians generally believe in a very small government. Standard views are that the free market is the best solution, drugs should be legalized, that cooperative, voluntary behavior can solve the social problems of the day.
Politically its represented by the Libertarian Party of which I'm a member.