A trainer is a program written to intercept and alter the memory addresses of games that are running in the background. Usually trainers contain such features as GOD MODE, UNLIMITED LIVES and others that sometimes aren't pre-programmed into the game by its designers. The most important thing to remember about trainers is that they usually have to be running in memory the entire time the game is loaded. Closing out the trainer will usually cause the cheats to become disabled (until the trainer is loaded again). Since trainers address specific memory locations, they are usually written to only support a specific version of a game. If you're having trouble getting a trainer to work, read our TRAINER TROUBLESHOOTING GUIDE which also includes instructions on how to properly use a trainer.
Basically, a trainer is a program that you run alongside a game that allows you to activate cheats by pushing certain hotkeys on your keyboard.