Clicking on a window under Mac OS X will bring the window and its application into focus. You might used to clicking the background window of an app, bring it on focus, in order to make the app window visible on screen. You can actually move a background window without giving it the focus. Simply Command-Click on the background window’s title bar and drag to move.
This is handy for example if you’re trying to perform a drag and drop of files from Finder into Mail compose window. While composing your email, simply Command-Click to arrange a Finder window until it is visible on the screen. This will save some clicks by keeping the email compose window in focus.