It's the print screen key. Hit the key, open up paint, then press ctrl + v. You should have your screen the way it was when you hit the key.
I dunno of any other use for it.
Edit: I found an update.
SysRq or SysReq (short for System Request) is a key on keyboards for PCs that has no standard use......
*en.wikipedia.org/wiki/SysRq