Chomsky thought that universal grammar is a special device of human brain which can help people learn language quickly. It is an unconscious and potential knowledge which exists in human mind without learning and determines the existing appearance of human language. Just because of this special device, the most stupid person can talk while the smartest monkey cannot speak. The basic concepts of universal grammar are principles and parameters. They reveal the commonalities and differences between natural languages respectively. Principles refer to the abstract grammatical property which can be applied to all human languages. A certain language may not have some principles, but there is no language that can violate these principles. Parameters reflect the differences between different languages.