What does that "tag" button do on the Wish List do? 

Nancy

Here's an explanation from the Help Center:

Book Tags allow you to mark books any way you like. 

• You can tag a book as "great" or "why would anyone read this?"
• You can apply any number of tags to the same book.
• You can view all the items in the database bearing a particular tag by clicking on the hyperlinked tag wherever it appears on a book listing.
• Others can see your tags.
• When you view a book you will see ALL of the tags you have applied to it, as well as the most commonly-applied tags for the whole club.  Less-commonly applied tags will not necessarily show, especially if a book has had many different tags applied to it.