The tag feature allows you to add tags (descriptors or subject headings) of your own choosing to a record. The idea is that it will make it easier for you to find the records again if you want to. Tags are not indexed for searching via the general search input box, but if you pull up a record that has a tag on it you can click on the hyperlinked tag to execute a search for anything else in the database to which someone has assigned the same tag. Note that the database to which the previous sentence refers might be the union catalog or might be the concatenation of all local catalogs: a tag search in one I-Share library's local catalog will retrieve records from any other local library catalog to which a user has happened to attach the same tag.
When you mark an item as a "favorite," you have the opportunity to add tags, but it is not necessary for an item to be a favorite in order for you to add a tag. You must be logged in to assign tags; tags are associated (in the background, not publicly) with the user who entered them.
On the single-title Record page, anyone can see all tags that have been added to a record, and from the Record page anyone can click to execute a search on any of the tags that were added in the process of making a record someone's favorite. On the Favorites page, you see only your own tags for a title.
Due to an internal constraint in VuFind, once the same tag has been attached to approximately 200 records in the same database (that is, to 200 records in the union catalog or to 200 records across all the local catalogs), clicking on a tag in one record will no longer retrieve the other records tagged the same way: it will instead generate the misleading message, "Your search - - did not match any resources."