An unflattering description of RPGA campaigns (usually used to describe Living City). In many of these campaigns everything of value (magic items, antiques, special equipment, etc.) is described on a certificate (a strip of paper). Certs are pre-printed with the name of the thing being certified, any special properties or rules that apply to the item and the name of the module the cert was issued in.
When a cert is issued the player fills in the name of the character that owns the certed item. Then the judge who ran the module fills in the name of the convention or game day, signs and dates the cert. Often conventions will stamp or sticker certs to provide additional authenticity. Munchkins (of which there is no shortage) usually play RPGA campaigns in order to collect the most (and most valuable) certs.
Certs can have value either through physical rarity ("only two of these certs were issued"), through in-game fantasy value ("this item is worth 400gp -- it says so on the cert") or through demand ("ever since HQ changed the rules about something this cert is very useful to characters who must deal with those particular rules").
See also: collectible book game and collectible card game