What is indexing?
Indexing is the art of organizing information. Indexes point readers or users to the information they are seeking. The most common type of index is the back-of-the-book index. Many books contain indexes and many don't. Pick up a book off of your bookshelf. Turn to the back of the book. Does this book have an index? If not, can you easily find information you are looking for within the book? If so, are you able to easily find the information you are looking for within the book?
A good index allows readers to quickly find what they are looking for without sifting through many pages of the book. The indexer is responsible for reading through the manuscript and not only selecting key terms within the book, but also selecting key concepts. Once the indexer has a good grasp of the book's terms and concepts she then creates and organizes an index suitable to the book's audience.