Session ID's are used by Dynamic webpages, these are webpages which will use a specific one off url address for each individual visitor. The url will look something like this,

http://www.mysite.com/cgi-bin//seriesHomepage.do?service=UK&SERIES_ID=113974

Session ID = _ID=113974 part of the above url

The numbers are created and tracked on a daily/weekly/monthly basis by the use of cookies. If your browser has not enabled cookies it will probably not beable to load up the webpage.

What are the use of 'session' ID's?, they enable webmasters to serve users with custom content, and can also help to defeat browser caching. The major problem is that most search engines like Google have major problems indexing dynamic content because it alters for each user and they cannot find a static location for content easily. Quite often it looks to search engines that numerous duplicate pages have been created and can ban the site for this reason.