What is a Flat Taxonomy
Flat taxonomy is classification in which the taxa have equal weight. A taxon cannot have child taxa.
Sitefinity CMS provides API for working with flat taxonomies.
Flat Taxonomy and Tags
The Tags functionality fits in the specification of Flat Taxonomy, therefore The Tags System is implemented on top of Taxonomies:
Tags implemented as Taxonomy
Tags are equal members of a single structure, just like the items in a Flat Taxonomy.