Well, a GameShark is a game-hacking device. Inputting certain codes allows you to override the game's coding and do things the game isn't supposed to let you do... have infinite money, for instance, or walk through walls.
There is a downside to this; overriding the game's coding causes it to screw up on occasion, so you're taking a big risk there. The walk-through-walls hack should be self-explanatory on what could happen, but the infinite money code could have a similar effect to what happens in HM: Another Wonderful Life when you have too much money: The game won't register you as having any money at all, and then you're screwed completely.
GameSharks cost around $20-$30, but for a GBA game they shouldn't be that much... however, they'll be hard to find nowadays.
As for question 3, you need to be playing your GBA game on a GBA system and be using a GBA GameShark. A DS Action Replay/GameShark won't be able to hack a GBA game.
**EDIT**
Well, after some rethinking, I gathered that maybe you won't need a Game Boy Advance to use the GBA GameShark since it basically runs the game like a normal GBA game when you select it on the start-up screen... but if you have a GBA, I recommend using it.
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So yeah... any more questions, lemme know.
« Last edited by Ligress on Jul 19th 2007 »