In the original Star Trek series, this is the black box with the flashing light that Yeoman Rand would hand to Captain Kirk and which Kirk would sign.
It could easilly be said that Star Trek inspired the invention of the PDA and the stylus as seen on the show.
In TNG (The Next Generation) it looks more like a large Personal Digital Assistant. In later series it is used much more heavily and is used to contain any source of reading material such as novels, journals or technical update manuals.