A key fob is a generally decorative and at times useful item many people often carry with their keys, on a ring or a chain, for ease of tactile identification, to provide a better grip, or to make a personal statement. Key fobs are often called "key rings" or "key chains" in colloquial usage. The word fob may be linked to the low German dialect for the word Fuppe, meaning "pocket."

Fobs vary considerably in size, style and functionality. Most commonly they are simple discs of smooth metal or plastic, typically with a message or symbol such as that of a logo (as with conference tchotchke) or a sign of an important group affiliation. A fob may be symbolic or strictly aesthetic, but it can also be a small tool. Many fobs are small flashlights, compasses, calculators, penknives, discount cards, bottle openers, or USB flash drives.

Electronic key fobs are used for remote keyless entry systems on motor vehicles. Early electric key fobs operated using infrared and required a clear line of sight to function. These could be copied using a programmable remote control. More recent models use challenge-response authentication over radio frequency, so these are harder to copy and do not need line of sight to operate. Programming these remotes sometimes require the automotive dealership to connect a diagnostic tool but many of them can be self-programmed by following a sequence of steps in the vehicle and usually requires at least 1 working key.

Key fobs are increasingly used in apartment buildings and condominium buildings for access to common areas (ie. lobby doors, storage areas, fitness room, pool). These usually contain a passive RFID tag. The fob operates in much the same manner as a proximity card to communicate (via a reader pad) with a central server for the building, which can be programmed to allow access only to those areas in which the tenant or owner is permitted to access, or only within certain time frames.

Security tokens from RSA Security designed as key fobs.Telecommuters may also use an electronic device known as a key fob that provides one part of a three way match to log in over an unsecure network connection to a secure network. This kind of key fob may have a keypad on which the user must enter a PIN into to retrieve an access code, or it could be a display-only device such as a VPN token that algorithmically generates security codes as part of a challenge/response authentication system.

Keychain

A keychain or key chain is a small chain, usually made from metal or plastic, that connects a small item to a keyring. The length of a keychain allows an item to be used more easily than if connected directly to a keyring. Some keychains allow one or both ends the ability to rotate, keeping the keychain from becoming twisted, while the item is being used. A keychain can also be a connecting link between a keyring and the belt of an individual. It is usually employed by personnel whose job demands frequent use of keys, such as a security guard, prison officer, janitor, or retail store manager. The chain is often retractable, and therefore may be a nylon rope, instead of an actual metal chain. The chain ensures that the keys remain attached to the individual using them, makes accidental loss less likely, and saves on wear and tear on the pockets of the user.

A keychain can also be a short chain used to link together a number of keys or other items. Sometimes keychains are hung on walls.