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. The word fob may be linked to the low German dialect for the word Fuppe, meaning "pocket", however, the real origin of the word is uncertain. Fob pockets (meaning 'sneak proof' from the German word Foppen) were pockets meant to deter thieves. A short "fob chain" was used to attach to items, like a pocket watch, placed in these pockets. 

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 trinkets) 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, security tokens and USB flash drives. As electronic technology continues to become smaller and cheaper, miniature key-fob versions of (previously) larger devices are becoming common, such as digital photo frames, remote control units for garage door openers, barcode scanners and simple video games (e.g. Tamagotchi) or other gadgets such as breathalyzers.

Some retail establishments such as gasoline stations keep their bathrooms locked and customers must ask for the key from the attendant. In such cases, the keychain has a very large fob to make it difficult for customers to walk off with the key.