Second language acquisition is the process of learning a second language in addition to one's native tongue. It involves becoming proficient in the new language through both linguistic and communicative competence. Theories of second language acquisition study how languages are learned, whether through formal classroom instruction, immersion in social situations, spontaneous everyday communication, or guided mastery of language systems. Key concepts in SLA research include the distinction between acquisition through subconscious exposure versus conscious learning, as well as the difference between competence in a language's internalized grammar and actual performance using the language.