The Acquisition Learning Hypothesis

The acquisition-learning distinction is perhaps the most fundamental of all hypotheses proposed by Stephen Krashen. It states that adults have two distinction and independent ways of developing competence in a second language.
The first way is language acquisition, a process similar, if not identical, to the way children develop ability in their first language. Language acquisition is a subconscious process; language acquirers are not usually aware of the fact that they are acquiring language, but are only aware of the fact that they are using the language for communication. The result of language acquisition, acquired competence, is also subconscious. We are generally not consciously aware of the rules of the languages we have acquired. Instead, we have a “feel” for correctness. Grammatical sentences “sound” right, or “feel” right, and errors feel wrong, even if we do not consciously know what rule was violated.
Other ways of describing acquisition include implicit learning, informal learning, and natural learning. In non-technical language, acquisition is “picking-up” a language.
The second way to develop competence in a second language is by language learning. We will use the term “learning” henceforth to refer to conscious knowledge of a second language, knowing the rules, being aware of them, and being able to talk about them. In non-technical terms, learning is “knowing about” a language, known to most people as “grammar” or “rules”. Some synonyms include formal knowledge of a language, or explicit learning.
Some second language theorists have assumed that children acquire, while adults can only learn. The acquisition-learning hypothesis claims, however, that adults also acquire, that the ability to “pick-up” language does not disappear at puberty. This does not mean that adults will always be able to achieve native-like levels in a second language. It does mean that adults can access the same natural “language acquisition device” that children use. As we shall see later acquisition is a very powerful process in the adult.
Error correction has little or no effect on subconscious acquisition, but is thought to be useful for conscious learning. Error correction supposedly helps the learner to induce or “figure out” the right form of a rule. If, for example, a student of English as a second language says “I goes to school every day”, and the teacher corrects him or her by repeating the utterance correctly, the learner is supposed to realize that the /s/ ending goes with the third person and not the first person, and alter his or her conscious mental representation of the rule. This appears reasonable, but it is not clear whether error correction has this impact in actual practice.
Evidence from child language acquisition confirms that error correction does not influence acquisition to any great extent. Brown and his colleagues have shown that parents actually correct only a small portion of the child’s language (occasional pronunciation problems, certain verbs and dirty words). They conclude from their research that parents attend far more to the truth value of what the child is saying rather than to the form. For example: “Her curl my hair” was approved because the mother was in fact curling Eve’s hair. On the other hand, “Walt Disney comes on Tuesday” was corrected, despite its syntactic correctness, since Walt Disney actually came on television on Wednesday. Brown conclude that it seems to be “truth value rather than syntactic well-formedness that chiefly governs explicit verbal reinforcement by parents which renders mildly paradoxical the fact that the usual product of such a training schedule is an adult whose speech is highly grammatical but not notably truthful”.

Taken from: Krashen, S. (1982). Principles & Practice in Second Language Acquisition. Pergamon Press Inc. p.10-11