Input devices such as a touch screen, keyboard, or microphone let you enter information to your handheld and interact with the info already there. Most palm-size handhelds use a touch screen, on which you tap letters on a virtual keyboard on the LCD, using a stylus or a pen. Or you might use a keypad, on which the keys are in alphabetical and numeric order, similar to the ten-key calculator pad on your PC keyboard. In addition, other keys will operate specific functions.
If the handheld you choose comes with handwriting recognition software, you can use a stylus or pen to write letters on an LCD touch screen tablet, which is quicker than tapping. You'll have to follow some writing rules to get the best results, but the effort is worth the subsequent convenience. Even better, you can get natural handwriting recognition software and train the software to read your own writing.
In order to actually type, you'll have to get an external keyboard or move up in size to a handheld that will accommodate a keyboard. It won't be as roomy as the one on your PC, but with a QWERTY design you'll be able to touch type.
Finally, voice recognition software allows you to give voice commands to your handheld to input data and operate certain functions.