"Game-used cards" feature pieces of equipment used in an event. Most popular are small swatches of uniforms or pieces of bats that are embedded into baseball cards. More unique game-used cards feature larger swatches, bat knobs, numbers or letters from jerseys, gloves, bases, and seats from old stadiums.
Game-used items, such as bats, gloves, batting gloves, caps, jerseys, bases, pants, locker name-plates, line-up cards, etc. are often sold by professional teams. It is imperative that, unless you've gotten the item in person, the item is authenticated. So-called "game-used" items with no authentication are extremely easily faked. The best authenticators for game-used items are MLB and Steiner.
The difference between game-used and game-issues are as follows: game-used has been used in/during an official baseball game. Game-issued merchandise is issued (usually by the team) for the purpose of possible use in an official game, however it is not necessarily ever used in the game. Game-issued items are usually uniforms (jerseys, pants, caps).
Pictures of my game-used cards may be found in my photobucket.