A pen tablet is an input device that allows a user to draw or write directly onto a flat surface with a stylus and transfer an image of the drawing or handwritten text to a computer. When a compatible program is active, it displays the words or pictures as soon as you create them. You can also use a tablet as an alternative to a mouse by moving the stylus to move the cursor, and tapping the stylus on the tablet's surface to click an icon or other object on the screen.
A digital notepad is similar, but it can store and display the drawings or text within the notepad and allow the user to transfer them to the computer later. Therefore, you can use a notepad just as you would use pen and paper to make handwritten notes at a meeting or lecture without the need to carry a computer.
Many tablets and notepads use a grid under the writing or drawing surface with a unique address for each point on the surface that corresponds to a pixel on the monitor screen. When the tablet or pad detects pressure from the stylus at a specific location, it either sends that information to the computer through a USB port or stores it in local memory inside the notepad for later transfer. After the CPU receives the information, it uses instructions from the device driver to interpret the location of the stylus (and a command, if any) and relays it to the active program, which displays a mark on the screen or performs the command.
Tablet computers do something similar, but they use a special monitor screen that has a layer beneath the surface that can sense stylus pressure rather than a separate drawing pad. A special version of Microsoft Windows, called Windows XP Tablet PC Edition, includes the device drivers and other programs necessary to incorporate the Tablet PC functions into Windows.
Tablets and notepads are not the same as the touch pads that many laptop computers use as an alternative to a mouse. The touch pad is a separate surface; touch-sensitive notepads and tablets accept inputs directly onto the computer's display screen.
Tablets and notepads offer more precise control of a cursor for drawings and handwritten notes because the stylus duplicates the familiar motion of a pen, pencil, or brush. It's possible to use a pen tablet as a replacement for a mouse, but it's often easier to have both: a tablet and stylus for drawing, and a mouse for the usual cursor controls.