Texts belong to different 'registers'.  According to Michael Halliday and Christian Matthiessen (2004: 27), registers are 'ways of using language in different contexts'.  Register therefore describes the linguistic characteristics of a given genre.  For example, there are formal and colloquial registers used in official and everyday settings respectively.  In some cases, there can be register-variation within a single genre.  For example, newspaper reporting is a single genre but different newspapers may vary in register. 

Register is described in terms of the configuration of linguistic resources in lexicogrammar, which members of a 'speech community' associate with a given situation (Halliday 2007: 182).  According to Halliday and Hasan (1985: 38) three aspects of situation determine register: field, tenor and mode.  Field refers to the activity in which the text-producer is participating.  Tenor refers to the social relations held between text-producer and text-consumers.  And mode refers to the medium by which the text is produced.