Retail Card—these are cards that are sold to major retailers like Kmart. The cards will often have the name of the store printed on the card as well. You might even find a card from a now-defunct retailer.
Insert Card—these are cards that are inserted into packs at a staggered rate (like one card being inserted into every 24th pack). There is also a number on the back of every sports card. The number on the back of the insert card will be different than the normal set numbers. The normal set numbers will appear as 1-400 (or however many cards are in the set), the insert cards will have a number like ST1, or PL1. When you buy a pack, you never know what kind of insert card could be in there. There even could be a player who became much more famous later on.
Sell Sheets—these are not cards at all. They’re ads that are sent to distributors for cards that are for sale to the public. This would show what cards you could get in the set, and would show the players that are featured in the set. You could get these ads from a sports cards dealer for pretty cheap, or even free if the retailer is going to throw them away (it never hurts to ask them if it’s possible for you to have it). They’re also great to display along with a complete team collection!
So what’s the rarest card that you’ve ever found?