The term sitemap can have two slightly different meanings:
A) The sitemap for humans
A webpage to give your users an overview of your site.
This is what your example
http://sites.google.com/site/(name of the site)/system/app/pages/sitemap/hierarchy
is. Just paste it in the URL-Bar of your browser and see yourself.
B) The sitemap for machines
This kind of sitemap is a machine readable (txt of xml) list of URLs
that comprise a website. It's the kind of sitemap that allows crawlers
to see what is on the site.
You can even have multiple sitemaps of this kind.
The reason for this is the same as why we don't have all source code in one file usually. It just easier to manage if you split the sitemap into multiple files.
In your example
http://sites.google.com/site/(name of the site)/system/feeds/sitemap
the "feeds" indicates that this is a sitemap which contains URLs for just the RSS feeds.
To learn what it contains you will have to take a closer look at it.
One way to do this is to download the file like this
wget http://sites.google.com/site/(name of the site)/system/feeds/sitemap
and open it in your favourite text editor.
At my site at google this file looks like this:
<?xml version="1.0" encoding="UTF-8"?>
It is nearly empty.
The reason is probably that I have just created the site and no feed entries exist.