Classic Shell is a suite of tools which makes Windows Vista, 7 and Server 2008 look and feel a lot more like Windows XP.
If you really don't like the modern Windows Start menu, for instance, then Classic Shell may appeal, as it brings back the much simpler XP version: a single list of options, and if you choose "Programs" then you see a folder for every application installed on your PC.
An Explorer plugin similarly replaces the new copy dialog with the older XP-version. It can show the free disk space and total size of the selected files in the address bar. You get a toolbar for some common operations (Up, Cut, Copy, Paste, Properties, Email), there are extra customisation options, you can disable the breadcrumb-style address bar, and more.
And Classic IE9 is a plugin for Internet Explorer 9 which shows the security zone and loading progress in the status bar, as well as adding a caption to the window title bar so you can see the complete title of the current page.
Don't want all of these? Then you can choose what you install, and of course you can always reconfigure your system after installation (the IE plugin can be removed just like any other, for instance). There are also lots of configuration options, and in some cases the standard Windows options are still available (hold down Shift and click the Start orb, say, and the regular Windows Vista/ 7 Start menu appears).
So, even if, say, you like the new Start menu and copy dialog, there may still be elements of Classic Shell which you find useful, and it's definitely worth a quick download, just to see what the program can do for you.