This is the 'manual' way to do this of course. Those of us using dynamic websites have to be a bit more clever, although automatic sitemap generators help. In Joomla! I use Joomap...
Anyway a great article to read to get to understand the importance of Sitemaps within Google!
by Shaluinn Fullove
Have you found it difficult to rank favorably for the keywords important to your business? Are all of your web pages visible in the natural search results? Here are five ways Google Sitemaps can help you drive more potential customers to your site and improve the visibility of your content.
Let's start off with a quick look at what Google Sitemaps is – and isn't. Google Sitemaps is a service that allows you to submit ALL of your pages to the Google index. It's particularly useful for making sure that dynamically generated URLs or pages that are not adequately linked to on your site get indexed. By submitting your URLs in a Sitemap, you help Google's web crawler do a more complete and efficient job of crawling your site.
But, it's important to note that submitting a Sitemap will not guarantee inclusion or influence your PageRank. It should act as a complement to compelling and useful content instead of a replacement of your SEO activities.
Conversion University - Sitemaps for Search Engine Optimization (SEO)