File extension PAGES are actually documents created by Apple Pages.  Apple Pages is a word processor and page layout application.  This document may contain basic text document or a complex multi-page brochure which can include other forms of data such as text, pictures, photos, tables, graphs, and charts.
The Apple pages which can be identified with having a file extension PAGES, is part of Apple’s iWork suite, which also includes Keynote (for presentations similar to Powerpoint) and Numbers (for spreadsheets similar to Excel).
Apple pages has been compared with MS Word. Although in terms of usage, MS Word clearly outnumbered Apple Pages users, some might say (specially the Apple fanatics) that Apple pages is far superior than word.
It has also been argued that the PAGES format is far better than the standard DOC format. However, since DOC format has already a widespread use, it has become the defacto standard in document format.