Halliday & Matthiessen (2014: 55):
While a description is an account of the system of a particular language, a theory is an account of language in general. So we have descriptions of various languages such as English, Akan and Nahuatl; but we have a theory of human language in general. This introduction to (systemic) functional grammar is both an introduction to the general theory of grammar and to the description of the grammar of a particular language, English.