Sitemap has 2 variants: HTML and XML.
1. HTML sitemap helps vistors to easily navigate a website. Usually you can find it at the bottom of the site. For example Webmaster Serve has HTML sitemap.
Here is a video that explain importance of HTML sitemap:
2. XML sitemap helps Google bot to crawl your website faster in a more effective manner. It is very good way to crawl undiscovered urls. Matt Cutts said that sitemap.xml is not important for every site but if you have many pages and want they to be indexed correct, you should create XML sitemap.
After submitting sitemap throgh Google Webmaster you can easily look from where traffic is commening, check about information crawling and indexing.its an easy way for webmasters to inform search engines about pages on their sites that are available for crawling.
Sitemaps are a way to tell Google about pages on your site we might not otherwise discover. In its simplest terms, a XML Sitemap usually called Sitemap, with a capital S is a list of the pages on your website. Creating and submitting a Sitemap helps make sure that Google knows about all the pages on your site, including URLs that may not be discoverable by Google's normal crawling process.