APPLIED LINGUISTICS aims at solving language-related problems and addresses real world concerns through the exploration of language in use. 

MOTHER LANGUAGE or FIRST LANGUAGE (L1) language that one acquires during childhood 

SECOND LANGUAGE (L2) a language that one acquires and learns in the country where this language is spoken

FOREIGN LANGUAGE a language that one acquires or learns in a country where this language is not spoken.

He talked about the the relation between comprehension -> Production, which means that a child for example, after birth, he/she starts listening and trying to comprehend a language before reaching that level of producing language.
BEHAVIORISM is another concept that we have analysed, ((Definition not given by the professor: It is a theory that human and animal behavior can be explained in terms of conditioning, without appeal to thoughts or feelings, and that psychological disorders are best treated by altering behavior patterns)). This theory account for *Describing (WHAT does language...), *Explaining (How does language ....), and about *Justifying (Why does language...). And it is the 1st school which tried to account for acquisition. we learnt also the relation STIMULUS -> RESPONSE. it is under the title called CLASSICAL CONDITIONING (a section from behaviorism) and it was proved by Ivan Pavlov in his famous experiment on his dog.

Classical Conditioning vs Operant Conditioning