Theory:

An index is a list of names or topics that are to be found in a book. It is a list arranged in alphabetical order at the end of a book.
It is mostly found in the end of a book and therefore is also called the back-of-the-book index. Its main purpose is to help a reader locate information present in the book easily. A book may contain several chapters, words, and topics. A reader may not want to read the whole book for a particular information. It is therefore easier to check the index, search for the beginning letter of the topic that one wants, and check for the page number, thereby reducing the work load.
  
The index of a book must not be confused with the table of contents which is usually given in the beginning of the book. This gives an overall view of the content of the book, in the order it is given in the book, whereas an index gives the names, places, dates contained within each topic thereby narrowing things down.
  
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Example of an index page