Promo: This usually refers to a card which is used as a promotional item. Hence the name "promo." This does NOT refer to unlicensed (Broder) cards.
These cards are usually handed out at large shows by manufacturers to allow the public a preview at an upcoming issue. Although one recent trend has been to insert promos into packs.
The glory days of promo cards were the early 90s. During the 1991 National Sports Collectors Convention in Anaheim, California, there were reports of some promos selling for literally hundreds of dollars within seconds of given away for free at the door. There were also reports of some buying as many as 50 tickets, just so they can keep going through the line to stock up on promos. Classic was famous for putting out TONS of promo cards. "Promo-mania" died around 1993 and most promos of the era that once sold for $100, now sell for a buck or two.