Language acquisition, as opposed to language learning, is a subconscious activity. 

INNATISM

A person is predisposed to the "feel" of a language: in other words, there is an innate capacity that allows a person to "pick up" a language.  This acquisition is acquired from from biological forces, then [language aptitudes]. Such a natural "feel" for a language does not include any conscious acquisition of rules of grammar or syntax, for instance.

INFLUENCE OF THE ENVIRONMENT

In addition to the innate aptitude of individuals, there is an interaction with one's environment which also assists an individual in acquiring language.  This area falls into the field of operant conditioning in which one is exposed to certain stimuli and then becomes conditioned to react in a certain way. This is also a non-cognitive method of acquiring knowledge.  Exposed to certain language patterns repeatedly, the individual then acquires these patterns and words, for instance.  That is, the individual seems to just "pick up" this language knowledge.

One method is "social interactionism." Here, the relationship between adults who have language and children who do not is paramount. Psychologist Jerome Bruner made the theory popular in the West, but it actually originated in Russia by another psychologist, Lev Vygotsky. Social interactionism privileges feedback between child and parent, or another adult. The adult both teaches and corrects the child as they acquire new words. 

A vastly different language acquisition theory is "emergentism." Emergentism postulates that language development is largely biological and that nature plays as great a role in language acquisition as does nurture. Without these automatic biological triggers, language development is not possible.