Pocket fluff is a strange and intriguing material often found in pockets, purses, sofas, and other fabric orifices. The mechanics of how it works are not entirely understood. (Dude, pocket fluff mechanics 101! Sign me up!)
Pocket fluff is formed within the pockets of, say, a pair of denim trousers, over the course of the day they're worn. It seems that whatever objects are stored in the pockets of the trousers each leave behind a residue or resin of some sort, be it money, keys, mobile phones, pagers, weaponry, or combustible items like cigarettes, foodstuffs, or packets of McDonald's Fancy Ketchup®. This residue or resin, by itself, is indetectable. However, once the wearer of the trousers removes the trousers and washes them, pocket fluff is formed during the washing/drying process by the residue or resin.
The result is often vaguely denim-colored and appears as though it may have once been paper of some sort, though in general it is too soft and too granulated to have been paper, but nothing else really fits the description. Therefore, any pair of trousers that have been washed (i.e., not new) will have pocket fluff in their pockets. The same goes for purses, various types of hats (fluff infestation of hats has not yet been tackled by the scientific community), the exhaust vents on computer cases, and that compact area of most sofas between the lower cushions and the frame, where other strange items are often found, not just pocket fluff.
Most people simply discard their pocket fluff upon their discovery of its existance in their pants (etc), though I have met at least one OCD geek who saved his in a cardboard box that he kept in one of his kitchen cupboards.
Also, pocket fluff played an important role in the computer adaptation of Douglas Adams' The Hitch Hiker's Guide To The Galaxy, released in 1984 by game publisher Infocom. The player had to collect four different kinds of fluff throughout the course of the game -- pocket fluff, handbag fluff, satchel fluff, and cushion fluff.