Years back, the traditional way to get your site indexed on Google was to wait.,, and wait.. and wait. It can take Google months to completely index a site, and then only if there are other sites that link to it. Google has launched a new system allowing you to tell Google about your site.
With a Site Map, you get to tell Google what pages, images etc. are on your site, and Google will use this information to index your site in days, instead of months.