Dear friends,
For your website to make the best performance in the search engines you need them to be aware of all of the pages on your website.  The best method of letting Google know about all of the pages on your site is to create a Site Map.
What is a Site Map?  Well, it is basically just a list of all of the web pages within your site.
What you want to say about it?

Site map is list of web pages at single web pages that can index all the web page at same time. It can helping to increase indexing rate of web pages.

Sitemap are of two types -

One is HTML sitemap

next one is XML sitemap

but we  know that XML sitemap is important for search engine point of view.

In seo sitemap is really very helpfull for search engine as well as users. XML sitemap is helpfull for search engines so that they easily indexed your site web pages.
HTML sitemap is helpfull for users to easily navigate your site links .

Sitemap as name suggest it's a list of all web pages of website which are accessible by users and search engine crawlers. Using sitemap website user and crawlers can easily access whole website pages and also easily navigate throughout the website. There are two types of sitemap one is XML sitemap and second is HTML sitemap. Generally XML sitemap is designed for search engine crawlers and HTML sitemap is used for users.

Check out the following link to get more detail about Sitemap.

http://en.wikipedia.org/wiki/Site_map

Google Sitemap is a way to tell Google which pages of your site you want to index. You just write the url in a text document (you can do a much sophisticated by using a xml document), name it "site-map.txt" and upload it to your root directory. That way you can maintain your list of url, and if you have to submit them again, it will be easier instead of going to the Google "submit-a-link" page and do it for every url.

If you don't have XML sitemap for your website then you will miss out some of the benefits like:

- can't get a better crawl from Google, since you have not told it which pages to crawl
- can miss reports on how you get traffic from Google
- will miss report on how your page is viewed by the Googlebot

Those who need reminding or educating here is why sitemaps are important. Google Sitemaps was officially started in June 2005. XML-Sitemaps.com provides an online Google sitemap generator that creates XML, HTML and text sitemaps and also helps you find and fix broken links on your website.

According to me its very important to have Site Map on our site and its important to have XML site map also its very hard to find that kind of information by site map.

Hello.....
Sitemaps are particularly helpful if:
a)Your site has dynamic content.
b)Your site has pages that aren't easily discovered by Googlebot during the crawl process—for example, pages featuring rich AJAX or images.
c)Your site is new and has few links to it. (Googlebot crawls the web by following links from one page to another, so d)if your site isn't well linked, it may be hard for us to discover it.)
e)Your site has a large archive of content pages that are not well linked to each other, or are not linked at all.

You can say without site map SEO in incomplete.

Site map is really important for both users and search engine crawler. It improves the readability of a site and increases the possibility of getting a top rank in the SERP. Site map helps the crawler to index your all web pages faster. Website page rank depends on quality of backlinks and quality of content on the web page. You should try to make quality and relevant backlinks from high PR websites... Its really will help you.

Sitemap contains all website pages so it is very easy to navigate to any of the pages from sitemap. There are two types of Sitemaps used, one is HTML sitemap and second is XML sitemap. XML sitemap is generally used for Search Engine, from XML sitemap search engine can crawl all your website page while HTML sitemap is used for Users, by visiting it user can navigate to any web page from sitemap.
So Sitemap is a very important part of Search Engine Optimization.