Since the Apple iPad exploded onto the scene in January it might seem that tablets are a brand new idea. However, some tablets have been used in recent years where a portable handheld device operated with a pen or stylus could do the job. This would include medical applications where they could replace a chart or possibly warehouse and delivery functions.
The tablet concept, however, is really quite old. In fact the "Dynabook" was created by Alan Kay in 1968. Kay wanted to make “A Personal Computer For Children Of All Ages". Read more of the details here. Alan Kay and collaborators at Xerox PARC are credited with designing many of the elements of modern computing including the mouse and the graphical user interface nearly forty years ago.
And, as with most breakthroughs, there were a lot of trys and a lot of failures preceding any successes. Read A Brief History of Unsuccessful Tablet Computers.