Normally, the cursor keys change the cursor position. When Scroll Lock is on, the cursor keys move the view instead. In other words, Scroll Lock causes the cursor keys to act like they do when viewing an HTML page in Firefox and IE. Spreadsheet software usually support it.
Re^2: The most useless key on my keyboard is:
Replies are listed 'Best First'.
Wow. That's so cool. I was going to vote for ScrollLock but read the replies before voting. I'm glad I did. This sounds like a key I'll have much use for, if the things I use actually support it.
I've used PrtScr several times in the last few weeks (it stuffs the current screen contents into the paste buffer so I can crop them and include them in e-mail to managers who like their facts in the form of pretty pictures). :)
Just for the sake of tradition, I'll note that the best answer was omitted. NumbLock, which should always be "on" (unless you have a miniature keyboard as part of some miniature device, of course).