Sorry if this is a stupid question. I just started studying linguistics today, so I'm kind of blundering through the subject at this point. I've been reading about various basic linguistic concepts such as dialect, discourse, and register. The articles I've read (mostly on Wikipedia) explain the difference between a dialect and a register or discourse fairly clearly, but I can't seem to find any explanation as to the difference between a register and a discourse, or even if there is one.
A register is usually a concept of social usage and is a property of a word or a phrase (e. g. a colloquial register, a high register, a low register, etc). A register by itself is not communicative and is dependend upon a social value of a communicative unit (a word or a phrase) attributed by a language speaker. It depends on sociolinguistic hierachy and the thing said (e.g. formal greatings, informal greatings and, in some languages, even swaring greetings are the different registers of greetings. Some languages, like Tibetan, Japanese, Tamil or Russian, have different words and/or grammars for different registers).
A discourse is a narrative, a text or a metatext as a stream of communicative units. It is therefore communicative by itself (and by the origin of the term), interdependent, based upon the things unsaid and incorporates various registers. E. g. different types and modes of greetings listed above produce a greeting discourse in a specific language.
Also, the meaning of register units is usually more permanent as compared to that of discourse units.