“A Cinderella stamp is any non-postage stamp. The oft-neglected stepchild of the postage stamp, a Cinderella may look like a stamp, but it won't carry the mail. The category includes locals, labels, tax stamps, fiscals, poster stamps, charity seals, forgeries, fantasies, phantoms, revenues, etc. Some are more elaborately designed than the postage stamps they imitate. The hard-core philatelist scorns any stamp that didn't carry the mail, but others find these philatelic by-ways fascinating and rewarding. Philatelic Exhibition Seals are a popular sideline among stamp collectors. The heyday for Cinderellas in the U.S. was the 1920's and 1930's, when many beautifully designed engraved examples were produced…”
Read more on Cinderella stamps here and here.