Tags
Tags categorize documents and specify how they should be handled. There are two types of tags: informational and security tags.
Informational tags
Informational tags provide more information about a particular document. The informational tags assigned to a document can be a good way to indicate the type or content of a particular document.
You can use informational tags to categorize documents in situations where you don't want to use a template field. For example, you might want the ability to mark particular documents as high priority. You could create an "Important" tag, apply it to documents that should be reviewed, and remove the tag when the documents have been dealt with.
Security tags
Security tags prevent unauthorized users from accessing documents and/or folders associated with categories they do not have permission to access. If a user has not been assigned access to a security tag, they will not have access to entries that have been assigned that security tag. This is true even if they possess another security tag that has been applied to an entry; the document is only accessible if the user has all security tags applied to the entry. For more information on security tags, see Securing Documents Through Security Tags.
Since security tags are independent from the folder structure, you can use them to restrict access to specific documents without having to independently configure entry access rights for the document or move the document into another folder.