Categories, tags, and facets are the descriptors for topics, themes and areas of interests.
“Facets are categories into which the properties of an object or topic can be divided. These categories in turn contain ‘values’: the sub-objects by which the list of items can be filtered. A classic example is the classification of wine. There are many properties by which wines can be categorised: region, colour, type, vineyard and price, among others.”
(Jim Kalbach a.k.a. @JimKalbach ~ Creative Blog)