You have two methods for downloading your Apps with a Redeem Code. You can use your iPod/iPad or you can use iTunes on the computer that you sync the iDevices with. Free Apps do not use a redeem code, you can just "buy" them for $0.00.
The Redeem Code can only be used one time and will automatically start downloading the purchase app to your iPad. Once the Redeem code has been used, the app is forever associated with this AppleID account. It can not be transferred to another AppleID account.