Game used: Simply a piece of an item that has been used in an actual official game (allegedly), and put on a trading card. NASCAR trading card manufacturer Press Pass was the first to put out cards with pieces of material used in contests, using driver's fire suits, pieces of the car's sheet metal, and chunks of used tires. Upper Deck quickly followed with their first Game Jersey cards in 1996 UD Football. Since then, they have appeared in many of their sets in some form or other. Most other makers have since followed suit with at least a few cards, though UD is by far the leader in this trend. Uniforms, batting practice jerseys (how the hell are these "game-used?"), shoes, balls, bats, pucks, shorts, sticks, nets, gloves, hats, sweatbands, batting gloves, and even the floor of the Delta Center have all been shredded for these cards.
Premiums are often paid for swatches featuring two or more colors, and have some sort of evidence of usage (i.e. grass and soil stains, blood stains, and the like). Patches, such as pieces of a jersey's number and nameplate, are more desirable as they have multiple colors and are taken from prominent parts of players' jerseys.
In recent years, some have begun to question the true authenticity of the materials used on these cards. A couple of years ago, three separate incidents brought the genuine nature of game used cards into question.
1) For their 2000 Invincible baseball set, Pacific produced a bat card of Manny Ramirez that appeared to have a wegde of cork embedded between two pieces of wood. As you could well imagine, both Ramirez and the Major League Baseball Player's Association questioned whether the bat Pacific used was the real deal. Pacific later admitted that they acquired the bat from a third-party who would not vouch for it's authenticity. Not coincidentally, Pacific lost their MLBPA license shortly thereafter.
2) In 2000, Topps released a "Topps All-Pros" jersey card of Miami Dolphins kicker Olindo Mare. As the name suggests, each card came with the claim that the jersey swatch mounted on the card was part of an "Authentic Player-Worn Pro Bowl Jersey."
Jeff Novell posted this message to the r.c.c.d. newsgroup weeks after the 2000 Topps Football product was released, doubting the authenticity of the card's jersey swatch:
As some of our long time viewers know, I am very good friends with Olindo Mare. Before I told him about his 2000 Topps Game USed (sic) All-Pro jersey card, he showed me his (guess what)...
framed 2000 All Pro Jersey. I immediately asked him if that was what he wore at the game, he said yes, and I asked if he wore any other jersey than that one. He responded negatively.
All of the 2000 Topps "game used" all pro jerseys of Olindo Mare are bull----. I've seen the original.
3) Upper Deck inserted into packs of 2001 Sweet Spot baseball, the first ever game jersey card of Seattle Mariners rookie Ichiro Suzuki. The only problem was, that at the time of the product's release Suzuki had yet to actually appear in an official Major League game. But that minor detail didn't stop Upper Deck from stating on the back of the card, "On the front of this card is an authentic piece of a game-used jersey worn by Ichiro Suzuki in an official Major League Baseball® game." UD quickly went into spin control claiming that the jersey swatch on the card was indeed worn by Suzuki. But, it was worn during a 1999 spring training game -- not quite an "official Major League Baseball® game."