This document summarizes different approaches to language description including classical grammar, structural linguistics, transformational grammar, language variations and register analysis, functional and notional syllabus, and discourse analysis. It discusses how the concept of language variation led to the development of English for Specific Purposes based on register analysis. Examples are provided to show how language varies based on context between informal spoken text versus formal written instructions. The document concludes that developments in language description are interrelated and distinguishing performance from competence is important in describing a language for learning purposes.