There are three concerns in the mythologies; language, culture, & ideology. The concern with language derives from Saussure. Culture from Levi-Strauss. Ideology from Marx. Each of the mythologies takes as its focus an aspect of culture (wine, film, travel guides, etc, etc). Each considers the language of that aspect of culture (object or concept) and how it communicates with us (how it structures how we read it, how it positions us, it's rhetoric). Each shows the ideological systems at work in this specific language of culture. Each mythology expresses this triangular relation of culture-language-ideology [Levi-Strauss-Sassure-Marx].
Typically the textual structure of each mythology follows the culture-language-ideology pattern (wine, the rhetoric of wine, the ideology of wine) which could almost be paraphrased as 'artefact x positions us in way y because ideology z'.
The concern of Myth Today is the lesson of all the Mythologies. It tries to answer the question 'what do all these individual mythologies add up to'. Barthes' answer is rather remarkable - and it is worth reading Myth Today carefully more than once for this reason - myth is the language of capital itself.
Each of the individual Mythologies is an example of this meta-language of capital [N/B not all of the mythologies are collected in the standard UK edition and there is another set of Mythologies available in an American translation so Myth Today is based on a very wide range of examples].
Myth Today could be seen as an attempt to outline the grammar of this language of capital. The culture-language-ideology triplet could be seen as the syntax of this meta-language of myth. The mythologies themselves are so many figures of speech within it. The specific aspects of culture discussed are its nouns. The rhetorics of culture illuminated in each mythology are the verbs of this meta-language. Culture, language, ideology are its registers...
It is worth remembering that this meta-language of capital has only one object, you the reader, and only one tense, the infinite present, because when it speaks it intends to weave us into a world that we are to understand we cannot change because it has always been this way and always will.