Yes, you can use it with the Leapster Explorer, the Leapster Explorer GS, and the Leappad's 1 and 2. Also, whatever apps you purchase you can use for up to three devices. With my three kids, this is perfect because I can purchase an app once and they can all have it on their different devices!
Yes! It just adds $20 to your LeapFrog Connect "App Center" account balance, and then you can use that towards LeapPad OR Leapster Explorer Apps -- just make sure you're in the right tab for your device.
I am 99% sure you can use this to buy apps in the leapfrog app store. Just make sure you are in the tab for Leapster Explorer apps and not Leappad apps. There should be a place to input the code, I think it says REDEEM code here.