Language is made up of systems and skills. The four systems of language are grammar, vocabulary, phonology and discourse.
The UCLES English proficiency exams explicitly test the four skills and also two systems, grammar and vocabulary.
In the classroom
Although most modern methodologies and approaches focus on communication and make little reference to systems, many course books still have a structure based on the systems and skills mentioned above. Discourse itself is normally not identified separately, but included in skills areas.