Tag Managing Thing provides a central interface for managing your tags (and categories!)
Splitting When you split a term, the things tagged with the selected term will be tagged with each of the split-terms. By default, the term being split will be deleted; however, if it is included in the list of split terms, then it will be retained.
Merging With merging, the selected terms will be merged into the term that is currently being edited. Any post which is associated with a merge term will become associated with the term that is being edited.
Switching When switching, if the term already exists, then the posts associated with the term in the current taxonomy will become associated with the term in the alternate taxonomy. If it doesn't exist, a new term will be created. In both cases, the term will be removed from the current taxonomy.