Universal Grammar is a linguistic theory developed by Noam Chomsky according to which all human languages are constructed on the same, abstract template, and that this explains why all normal speakers acquire their native language quickly and accurately.
In response to the logical problem of language acquisition, which challenges theorists to explain how all normal children are able to learn any first language in only a few years, Chomsky posited the existence of a Language Acquisition Device (LAD).
He suggested that the LAD was an innate, language-specific module which effectively pre-programmed children to learn language. The LAD operated by restricting the number and type of hypotheses learners entertain about the grammar, or rule-system of the language.
Chomsky developed the notion of Universal Grammar (UG) as a blueprint for the LAD. UG, or generative grammar, consists in an abstract description of the components and structure underlying all human languages.
Generative linguists seek to refine and test these abstract linguistic descriptions, while second language generativists research whether and how UG is available to learners of second languages.
Read more about Chomsky's Universal Grammar