An Index is a section that appears at the back of a book that details people, places, events, and concepts that were significant throughout the book. It is meant to be a quick reference for readers looking for a specific instance of a certain keyword.
The need for an index is largely dependent on your genre. Historical and Genealogical books often have indexes to help readers track the hundreds of people, places, or events that happen throughout the book. However, most books (Novels, Poetry, Art Books, Children’s Books) do not require an index. If you feel that your readers may need a reference but feel that a full index would be too much, you can include a list of locations or a cast of characters instead.
An index is formatted similar to a Table of Contents; the keyword(s) appear on the left and the reference number (typically a page number) appears on the right. Due to the large number of keywords that may be in your index, you may want to split your text into columns to accomodate.