A “shunt jumper”, probably most frequently just called a “jumper” but also known to just be called a “shunt”, is a small piece of plastic with two holes that are designed to nicely fit over some pins coming out of a circuit board, and a piece of conductive metal inside that small piece of plastic. The intended purpose is for the metal inside the plastic to cause an electrical connection between the pins. This is often used as a way to cause a circuit board to have a different effect than if the jumper was not connecting the pins. The precise, specific details on the effect will vary based on just how the circuit board is designed.
Wikipedia's page titled “Jumper (computing)” says “Jumpers have been used since the beginning of printed circuit boards.” They have frequently been used on computer motherboards, and on many older “hard drive” controller boards which are a part of hard drives that use a design called “integrated drive electronics” (“IDE”). However, newer hard drives have often used the SATA standard that resulted in not requiring any manual configuration with a jumper. While jumpers used to be somewhat common for some “expansion cards”, the use of jumpers has been entirely eliminated on many newer circuit boards, and greatly reduced for many motherboards. The key reason for this is that many newer circuit boards are now more configurable by using other methods, such as the “plug and play” technology which is commonly used by “peripheral systems interconnect” (“PCI”) expansion card slots, or “device driver” software that may be able to be configured after an “operating system” begins much of its startup process.
Jumpers are a standard small size, so motherboard pins will typically be placed a certain distance from eac other. Not all pins that are separated by that small distance are meant to have a jumper connected. Some pins like that may be intended to provide power to an LED, or power (and, thereby, audio data) to a “tweeter”-style PC speaker.
For now, this site is simply referring to some publicly-available pictures at Wikipedia's page titled “Jumper (computing)”.
Commonly, jumpers are about as tall as the pins they cover, although some are nicer and are roughtly twice as tall, with the top part of their height just being intended as a handle to be able to easily pull them from the motherboard.
It is quite common to leave a jumper into what the Wikipedia page called the “off” position, by having a jumper covering just one pin. Jumpers in such a configuration will typically stay put until they are pulled off of a pin, which makes this setup a convenient way to store a jumper that may be likely (or even rather certain) to later be necessary to connect pins at a later time. This technique has frequently worked perfectly well for many computers, including computers like laptops which may even have involved rotating the computer upside-down.
Sometimes, jumpers have been installed horizontally rather than vertically, connecting a couple of pins that don't need to be connected. This is also a technique that has been used to effectively store a pin, and may have been somewhat common for jumper pins on hard drive controllers, which are circuit boards that have often been part of a “hard drive”. However, this technique is not necessarily always safe. Details are likely to vary based on which pins are used, including varying based on which device is being used. So don't count on that technique unless you have seen some documentation that this is okay to do (which is probably a detail that has been documented on the stickers attached to some hard drives).
A tip: When installing a jumper, large fingers and small spaces can result in some reduced visibility and the installation may be questionable. After installing a jumper, it may make sense to pinch the “length” sides and then to pinch the “width” sides, trying to figure out whether you are just feeling plastic (from the jumper) or if you feel a metal pin. (If you feel a metal pin, that may indicate that the jumper didn't line up properly, and will need to be reduced.)
- Related Resource
For a similar topic, see: DIP switch.