Discourse has a structure

The way langauge is organised is called its discourse structure.

In written discourse: look at how it is put together. It may have an opning which leads the reader in to the text. The following sections may develop a theme or an argument. The final section may make a conclusion.

In spoken discourse: the structure is less organised. For example conversations are unpredictable and speakers often digress. This is because spoken language is spontaneous. Even spoken language has some structure though: usually there is an opening sentence: 

Hi how're you doing?

Fine thanks. How about you?

then there is turn taking where they talk about a topic or topics. e.g.

Yeah not bad, did you go to the match?

Nah, I was ill.I was in bed with flu all week. I was gutted i missed it, sounded like a great match.

That's a shame,I heard that Tom's got flu too…