XML sitemaps were introduced by Google in June 2005 as a way for website designers to show the Google spider all the pages in your website. Yahoo had its own format — urllist.txt — which served the same purpose but for their crawler instead.
In November 2006, all the major search engines , Google Yahoo and MSN, adopted the Google sitemaps.xml protocol as standard. Ask.com started using it about 6 months after that.
There are several free tool on the Internet that will generate a sitemap for you. If you site is over 500, then a lot of the online generators will not work. You need a Generator program. Sitemaps files are limited to 50,000 URL’s or 10 Megabtyes. This is easliy enough for 99.9999% of all websites. Sitemaps can also be saved and stored in the gzip format