What is a sitemap?

Some SEO tips can be hard to explain, but this is one of the few that is relatively easy to do and can be done manually with small or large sites. Create a sitemap of your website. There are a few good reasons to do this.

It allows easier indexing of your site by the search engines.
In other words, it helps the search engines find all the pages on your site. Some websites only have a few of their pages in the search engines and this can be due to poor linking, sparse navigation or a host of other reasons. A sure-fire cure is to have a sitemap links to all of the major sections of your website. If you have a small website, linking to all the pages is a good idea. Web-crawlers will thank you for making their busy lives easier.

It provides PageRank or link popularity to all pages it links to.
If you read about SEO then you have read how important it is to have high-quality links pointing to your site from other sites. While this is true, don’t underestimate your own internal links (as seen in last week’s SEO tip) pointing to your own pages. A sitemap can become another source of quality links with descriptive text for your own pages.

Sitemaps help with usability and site navigation.
You website users can now access any important page on the site with only two clicks. This allows for deeply nested pages to be found easier and not just by the search engine spiders but by people visiting your site. Making navigation easier by including a sitemap is just good business sense as well as SEO sense.

reference:
http://www.bigoakinc.com/blog/create-a-website-sitemap-seo-tip-week-6/