A touchpad is an input device commonly used in laptop computers. They are used to move the cursor, using motions of the user's finger. They are a substitute for a computer mouse. Touchpads vary in size but are rarely made larger than 20 cm² (8 in²). They can also be found in PDAs.
Touchpads commonly operate by sensing the capacitance of a finger, or the capacitance between sensors. Capacitive sensors are laid out along the horizontal and vertical axis of the touchpad. The location of the finger is determined from the pattern of capacitance from these sensors. This is why they will not sense the tip of a pencil or other similar implement. Gloved fingers may be problematic (such as in a cleanroom environment) but can sometimes work. Moist or sweaty fingers can be problematic for those touchpads that rely on measuring the capacitance between the sensors.
Touchpads are relative motion devices. That is, there is no isomorphism from the screen to the touchpad. Instead, relative motion of the user's fingers causes relative motion of the cursor. The buttons below or above the pad serve as standard mouse buttons. Depending on the model of touchpad and drivers behind it, you may also click by tapping your finger on the touchpad, and drag with a tap following by a continuous pointing motion (a click-and-a-half).
Some touchpads also have "hotspots": locations on the touchpad that indicate user intentions other than pointing. For example, on certain touchpads, moving your finger along the right edge of the touch pad will control the scrollbar and scroll the window that has the focus vertically. Moving the finger on the bottom of the touchpad often scrolls in horizontal direction.
Some touchpads can emulate multiple mouse buttons by either tapping in a special corner of the pad, or by tapping with two or more fingers.
Touchpads are primarily used in portable laptop computers, because the usual mouse device requires a flat table adjacent to the keyboard not always available away from the office. But touchpads have some advantages over the mouse, particularly that the pad's position is fixed relative to the keyboard, and very short finger movements are required to move the cursor across the display screen. Some computer users prefer them for such reasons, and desktop keyboards with built-in touchpads are available from specialist manufacturers.
Touchpads have also recently appeared in Apple's iPod. The main control interface for menu navigation in all of the currently produced iPods (except the Shuffle) is a touchpad (at first by Synaptics; Apple now manufactures that component itself). Creative Labs also uses a touchpad in their Nomad Jukebox Zen line with the Zen Touch, Zen Sleek (Photo) and Zen Micro (Photo).
The "trackpad" is Apple Computer's name for the touchpad. It was introduced in 1994 in the PowerBook 500 series, the first Apple laptop ever to carry such a device, and replaced the trackball of previous PowerBook models. Late generation PowerBooks and iBooks have two finger sensing capabilities, as well as the current MacBook and MacBook Pro model lines. These capabilities include the ability to option-click by tapping two fingers and the ability to scroll both vertically and horizontally at the same time by dragging two fingers (which is very useful when looking at a large photo, webpage, etc).
In 1989 Psion introduced their first full size laptop (Psion MC 200/400/600/WORD series) with a new mouse-replacing touch-pad. Although the Psion's was a tap-to-point design that did not catch on, however, the Apple stroke-to-point design did, so the Psion's system wasn't really a touchpad in term of how we know it today.