Language learning vs. Language acquisition

The capacity to acquire and use language is a key aspect that distinguishes humans from other organisms. While many forms of animal communication exist, they have a limited range of non-syntactically structured vocabulary tokens that lack cross cultural variation between groups. Now, how do humans learn or acquire language? What does learning a language means? What does acquire a language means? Both are processes humans use to know a language, but are they the same? How do they work?

Language acquisition is the process by which humans acquire the capacity to perceive, produce and use words to understand and communicate. This capacity involves the picking up of diverse capacities including syntax, phonetics, and an extensive vocabulary. This language might be vocal as with speech or manual as in sign. Language acquisition usually refers to first language acquisition, which studies infants' acquisition of their native language, rather than second language acquisition that deals with acquisition in both children (Plato) and adults of additional languages.

Second language acquisition or second language learning is the process by which people learn a second language in addition to their native language. "Second language acquisition" refers to what the learner does; it does not refer to what the teacher does. Second language acquisition research studies the psychology and sociology of the learning process. Sometimes the terms "acquisition" and "learning" are not treated as synonyms and are instead used to refer to the subconscious and conscious aspects of this process respectively.

The linguist Stephen Krashen differentiates in his theory two processes that happen when we “learn” a second language and calls them second language acquisition and second language learning. Acquisition is similar to the subconscious process that children already undergo when they acquire their first language. By contrast, second language learning is the conscious process of learning, for example, grammar rules. It leads to a conscious knowledge of the learned system.
Language learning focuses on developing the ability to communicate in a second language. Second language learning (SLL) is the term used by the linguist Stephen Krashen to refer to the process by which people consciously learn a second language. Krashen calls the subconscious process involved in learning a second language second language acquisition.

Reference:
Krashen; (1981); Second Laguage Acquisition and Second Language Learning; http://www.sdkrashen.com/SL_Acquisition_and_Learning/index.html