The Jewelers dictionary defines a Fob as a short chain, ribbon or other similar thing, fastened to a watch to aid in withdrawing it from a pocket in clothing.
I don't think this definition does justice to the world of Fob collecting. If I was to define Fob, I would say that it's part of the ritual of using a pocket watch. It's an object which may or may not have, personal meaning to the wearer, and that this object is attached on a watch chain to either anchor the chain into the second pocket of the vest, or to hang off the watch chain as a decorative piece of jewelry.
But how do you identify a fob? It can be something as simple as a thimble or a spool of thread, or a key, or as elaborate as an intricate hand made miniature automobile of precious stones set in gold or platinum. In all honesty, just about anything can be a fob. It's only parameters is that it's not too large for the watch chain or the pocket. It's only function is to add weight or decoration to a watch chain or watch ribbon. I've seen fob chains in gold, platinum, sterling, gold-filled, Boar's hair, leather, string, and even weaved out of human hair. In fact this was very popular in the Victorian times, a wife would use her own hair to make that special chain for her husbands cherished watch.