As smart as the Google spider is, it's possible for them to miss pages on your site. Maybe you've got an orphaned page that isn't in your navigation anymore. Or, perhaps you have moved a link to a piece of content so that it's not easily accessible. It's also possible that your site is so big that Google just can't crawl it all without completely pulling all your server's resources—not pretty!
The solution is a sitemap. There are three different kinds of sitemaps that we're going to cover in this chapter, each with a different purpose: