A sitemap is a map of your website for either one of two things;
1 - A Person to find his or her way around your site (HTML), or
2 - A search engine crawler or spider to find its way around your site (XML).
For example, the page you see listed above this paragraph is a html sitemap for you to read and hopefully, find your way around this site.
Box Off My Computer have another version of this sitemap called an XML sitemap which we have stored with our website files. We upload the XML version to our Google Webmaster account everytime we update so that Google spiders can crawl Box Off My Computer and find its way around, hence listing more of our pages in Google search results.
Google recommends having both an HTML and an XML sitemap for your site but if for some reason you only have time to create one, Google recommends HTML.
You can use a FREE Online Generator to create your own sitemap, this is good for XML sitemaps and HTML, please click here