Sitemap
A sitemap is a file containing information about a site's structure to help the search engines index the content. A sitemapis not really necesssary for small sites that have all the pages linked to each other via the navigation menu.
Larger sites that have more than one section and are built using more than one EverWeb project file would benefit from having a sitemap.
When a website is updated, a propely created sitemap which has been uploaded to the server and submitted to Google will get the new content indexed faster.
Format
Sitemaps can be created in various formats but perhaps the most common one is XML. A basic sitemap can be created and downloaded online for free.