With Internet Explorer 8 users are able to run multiple instances of the browser, completely separate, each in its own session. With the latest release of the Internet Explorer, the Redmond company aimed to simplify browser session handling, simplification which allows IER8 to move away from the traditional model of “session merging.” In this context, users will notice that IE8 comes to the table with an extra option in the File menu dialog in addition to New Window, New Tab, or Duplicate Tab. It’s called New Session, and when clicked it opens up a new window of IE8 that shares no cookies with the source window.

“Simply click the New Session item on the File menu, and a new browser window will open. The new browser window will not share session cookies with the original browser window, so you can log into Hotmail (and most web applications) as a different user. For command line junkies, you can run iexplore.exe with the -nomerge parameter,” explained Eric Lawrence, a security program manager on the IE team.

Most valuable links
http://news.softpedia.com/news/Internet-Explorer-8-New-Session-111037.shtml
http://wfdss.usgs.gov/wfdss/training_slides/QT_IE8_wfdss.pdf