GN4 provides two default ways to organize and retrieve your content by categories and keywords.
In blogger's jargon keywords are often known as tags - but not in this documentation, in order to avoid to confuse keywords with GNML and HTML tags.
Many content providers enrich the content with categories and/or keywords, so you can find such classification ready within delivered content. GN4 also provides a way to rename/remap original categories to your in-house categories.
The differences between categories and keywords is slight: categories are often considered as broad and encompass smaller, more defined topics described by keywords. You can expect to need no more than 50 categories, while the number of keywords is typically much higher.
See also
Configure refinements with custom lists