The navigation of a website is important in helping visitors quickly find the content they want. It can also help search engines understand what content the webmaster thinks is important.
Although Google's search results are provided at a page level, Google also likes to have a sense of what r apage plays in the bigger picture of the site.
All sites have a home or "root" page, which is usually the most frequented page on the site and the starting place of navigation for many visitors. Unless your site has only a handful of pages, you should think about how visitors will go from a general page (your root page) to a page containing more specific content. Do you have enough pages around a specific topic area that it would make sense to create a page describing these related pages (e.g. root page -> related topic listing -> specific topic)? Do you have hundreds of different products that need to be classified under multiple category and subcategory pages? Example:
The directory structure for our small website on website
A sitemap (lower-case) is a simple page on your site that displays the structure of your website, and usually consists of a hierarchical listing of the pages on your site. Visitors may visit this page if they are having problems finding pages on your site. While search engines will also visit this page, getting good crawl coverage of the pages on your site, it's mainly aimed at human visitors.
An XML Sitemap (upper-case) file, which you can submit through Google's Webmaster Tools, makes it easier for Google to discover the pages on your site. Using a Sitemap file is also one way (though not guaranteed) to tell Google which version of a URL you'd prefer as the canonical one (e.g. http://tutorial-trick-tips.blogspot.com/ or http://www.tutorial-trick-seo.blogspot.com/; more on what's a preferred domain). Google helped create the open source Sitemap Generator script to help you create a Sitemap file for your site. To learn more about Sitemaps, the Webmaster Help Center provides a useful guide to Sitemap files.