From Debtags point of view, a facet is a group of tags which describe one quality of a package. You can think of them as different points of view from which to look at the packages in Debian.
For example, there is a "role" facet which describes the role of a package in the system (program, documentation, examples...) and a "works-with" facet which describes what you can do with it.
Facets help to clarify the meaning of tags because they give them a context.
Since faceted tags describe packages from different point of view, the classification system has a remarkable level of detail.