Language Acquisition/Nature or Nurture/Competence and Performance
To understand the notion of UG, the distinction between competence and performance is essential. Competence is inside the language user's head, while performance is what the language user actually produces.
E-language and I-language
The notion of E-language ('externalised' language) is associated with Bloomfield's structuralist tradition of linguistics. Linguists who study E-languages collect utterances to create corpora. They then attempt to describe these data.
Chomsky believed the study of E-language was insufficient. E-language only looks at what people say. Any corpus, regardless of size, only contains a small percentage of what language users can produce. (This is because the number of sentences that can be produced is essentially limitless - we will look at this below.) Therefore, he proposed the notion of I-language, which describes the linguistic knowledge of language users. Language users know how to form sentences that conform to the grammatical rules of the language. They will not produces sentences like *Him is good very, and they can also tell that such sentences are wrong. This constitutes the linguistic intuition of language users.