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.