Free play refers to a specific type of promotion that is offered by some casinos, in which players are given a certain amount of money to use on certain games, usually slot machines. This money is not real money, but rather a form of "play money" that can be used to play the games but cannot be cashed out or withdrawn.

Yes, you run the risk of having your winnings "null and void" if you use another player's card. There are some instances of people being asked to leave the casino for using player's cards which are not theirs.