03-17-2011 06:42 PM
They are words that help people find your book when they do a search. For example: Romance, children's book, magic, parenting, self-help, inspirational.
03-18-2011 12:24 PM
Google some words that you think people might look up to find your book and see how far up the list each one is and how many searches each one has gotten. Also go to Amazon and see what some of their subcategories are and what people have tagged each one.