You may have a need to minimize the Ribbon to see a bit more of the screen in a program.  It is also possible that your Ribbon may dissapear and you do not know how to get it back.  In Office 2010 there are two ways to minimize/maximize the Ribbon.  In Office 2007 you will have to use a keyboard shortuct to perform this operation.  Both of these options are demonstrated in this video tutorial.