[Sean]

A scrap card is a card that, perhaps due to a printing flaw, was intended to be thrown away rather than distributed. They were taken home by the printers, who cut them by hand and gave them to their kids.
In the T206 set, all Brown Old Mill and all Blank backs are scrap. Also, many of the cards that are missing colors are scrap, including (I believe) a Wagner and two Planks.
Johhny has made a career of collecting such cards, and can explain this better than I can.
If you are interested, go to ebay and type in "T206 yellow." Dan McKee (a board member) has a huge set of missing color scraps that you can look at. Also, at T206Resource.Com there is a good article by Scott Forrest (another board member) on this subject.