In computing, a stylus is a pen-shaped instrument used with graphics tablets or devices that utilize touchscreen input. It is used to write or draw on a screen, similar to how you would on a sheet of paper. Unlike a pen, styluses have a plastic or felt tip depending on the its purpose and functionality. The picture example shows the Palm Z22 Stylus.
Which devices use a stylus?
Originally, the stylus was not commonly found or used with devices other than graphics tablets and PDAs. However, with the rising popularity of touch screens on mobile devices, many individuals like to use a stylus as a replacement for their finger. The reasoning varies, but it's usually for more accurate navigation or to keep oils from their fingers off of their device's screen.
What is the difference between passive and active styluses?
A stylus can come in one of two forms: passive (also known as a capacitive stylus) or active (also known as an active stylus or active pen). A passive stylus is not different than using your finger as far as the device is concerned. An active stylus (such as the one used with the Microsoft Surface) has an electronic component that interfaces with the device's touchscreen controller. They have a few distinct advantages: more accurate and finer lines, different levels of pressure sensitivity, and the ability to ignore input from your hand resting on the screen. The last feature is referred to as palm rejection.