A basic personal digital assistant can keep track of thousands of addresses and phone numbers, as well as your calendar.
More expensive ones can also store photos, play games, track expenses and much more.
Ones from Palm, Handspring and Sony have the Palm operating system. With those, you can't use regular writing. They're all single stroke characters. Ones from Casio, HP, Compaq and others use a form of Microsoft Windows and are called Pocket PCs. They're more like tiny computers and they can recognize regular writing.