Tags can be used to organise your content (eg. "List me all articles tagged 'important' or all images tagged 'vibrant').
Tags are words or short phrases. They cannot contains commas. Typically tags are more specific than more generalised 'categories' or page-level organisation.
They are also used for a non-hierarchical taxonomy (although it is possible to organise your tags in groups of related tags, and through a parent-child relationship).
Taxonomies or Tag Groups
Tags can be organised into groups (such as 'genre' or 'theme' or 'style').
Tags can be organised into a heirarchy by nominating a tag to be the'parent' of another tag.