For the purpose of Wikipedia maintenance and administration, tags are brief messages that MediaWiki automatically places next to certain edits (or logged actions). (Depending on the context, edits are sometimes called changes to a particular article or the contributions of a particular editor.) Edit tags are visible on histories, recent changes and certain special pages.
Edit tags can be used to detect potentially harmful edits or software bugs. Tags also are used to identify how someone made an edit, such as via mobile, VisualEditor, or GettingStarted. Edit filter managers, that is, users given access to the edit filters, can direct an edit filter to tag those edits that match the filter. Additionally, tags can mark edits which have been subsequently reverted. Without needing special access level rights, you can filter the lists of edits on the User contributions or Recent changes special pages by entering a tag in the Tag filter field. The list of established tags is at Special:Tags, and includes an interface for administrators to manage tags.
For each tag, the respective appearance and description system messages can be accessed from Special:Tags and modified by administrators. The prefix Tag(s): is automatically appended by MediaWiki. The tag appearance specifies how they read in the changes list. The tag description is used only for information purposes at Special:Tags. For consistency, the template ((tag description)) should be used for descriptions, with the edit filter(s) generating the tag specified if any. Tags should generally not be renamed from the edit filter interface when they have already been applied to edits, as this would create a new tag. So make sure to use the correct name from the start, or change the tag description itself. Currently, there's no way to reassign tags.
Tags with fewer than 5000 hits and not defined by extensions can be deleted by administrators (if no delete links are visible, then all the deletable tags have already been deleted). This action is irreversible, so caution is advised. When deleting a tag, the accompanying MediaWiki pages (tag description and tag name) should be deleted as well. It should be safe to delete tags that have not been used for several years or with few hits, although it is recommended to check the edit filter that added the tag to clarify its purpose and for an overview of the edits it was applied to. If an edit filter tag is disabled or even deleted, it still registers the tag so it must be removed from the edit filter first to become deletable. It is also possible to create and activate tags in preparation to bot tagging of edits, but tags cannot be added to revisions yet, so this functionality is currently of no use.