A site map augments, but does not replace, the normal menu system of a web page.
Building a good site map is critical for web pages which use JavaScript menus, because most search engines cannot follow JavaScript links. A Site Map will help those web pages to be indexed by the search engines.

Sitemap allows search engines to crawl the pages that are hard to find, pages that are not been linked to from other pages.

Sitemap for Users (is a bit different comparing with a Sitemap for search engines to crawl)
A site map can also be very beneficial to web users, because it will help them to understand what resources the web page has for them.

A site map should be very well organized and easy to understand. The design should be clean and simple.

How do we create one?

uCozers can create one in Control Panel but first he must install Site Search
To install Site Search go to CP » On the left bottom side click Inactive tab » Site Search
Install the module, a pop-out should appear check all the boxes that you want the search engine to crawl and than click Apply.

For those of you who already installed the Site Search you can update your Sitemap in
CP » Site Search » Update Sitemap file (button)

Example of Search Engine Sitemap and a Sitemap for users (it looks more like a menu) ex. http://searchsoa.techtarget.com/sitemap/0,296817,sid26,00.html

Why does Cubex have Site Map and Forum Sitemap?
During installation of the Site Search uCoz creates two Sitemaps one specifically for the forum and the other one for the rest of your modules that you had installed.

For those of you who want to know more on how the Sitemap works and how to manipulate I would suggest to see this website http://www.sitemaps.org/