Game shark is something you use on a console that hacks the game and lets you cheat.

The most recent ones use special memory cards to use them. You put in the Game Shark CD and memory card, choose the cheats, then swap the Game Shark CD out for your game disc.

The CDs that come with them have a lot of pre-installed cheats. You may need to go online and find new ones if they don't have what you're looking for, but no, you don't need a computer.

You can buy them at any store that sells video games. I think they're around $15-$20.