Krashen maintained that adult L2 learners have at their disposal two distinct and independent ways of developing competence in a second language: acquisition and learning. Acquisition is a subconscious process identical in all important ways to the process children utilize in acquiring their first language, and learning is a conscious process that results in knowledge about language. Acquisition comes about through meaningful interaction in a natural communication setting. Speakers are not concerned with form, but with meaning; nor is there explicit concern with error detection and correction. This contrasts with the language learning situation in which error detection and correction are central, as is typical the case in classroom settings, where formal rules and feedback provide the basis for language instruction.
Nontheless, for Krashen, it is not the setting per se, but conscious attention to rules that distinguishes language acquisition from language learning. In the natural setting, an adult can obtain formal instruction by asking informants about grammar and by receiving feedback from friends. Similarly, language can be acquired in the classroom when the focus in on communication --- eg. through dialogues, role-playing, and other forms of meaningful interaction.