Halliday & Matthiessen (2014: 59):
Word classes can be viewed ‘from above’ — that is, semantically: verbs typically refer to processes, nouns to entities and adjectives to qualities (of entities or of processes). They can also be viewed ‘from round about’, at their own level, in terms of the relations into which they enter: paradigmatic relations (the options that are open to them) and syntagmatic relations (the company they keep). On either of these two axes we can establish relationships of a lexical kind (collocations and sets) and of a grammatical kind (structures and systems).