Nintendo has created a point system in which points or credits act like a currency among different Nintendo consoles for buying games packages and other gaming content through Nintendo's e-shop. Wii and Nintendo 3DS points are collectively referred as Nintendo points.
So Wii points is the official currency on Nintendo e-shop for buying Wii games and stuff similarly 3DS points are used to buy 3DS games and other content. If you want to buy either Wii or 3DS points you have to buy a Nintendo card that is available on various offline game stores. You can also buy one online and get it delivered to your house. Best three websites from where you can buy a points card online are: Kmart (Kmart.com), BestBuy (bestbuy.com) and Gamestop (gamestop.com). You can also get Wii or Wii U points for free as many time you want here free Wii points.
After buying the card you will find a silver part on back of the card which you have to scratch. On scratching it will reveal a 16 digit code. To redeem the card just open the Wii or 3DS console go to your account then go to the redeem card section and put your card number. That's it your points are available on your console. Nintendo 3DS and Wii points are neither transferable nor interchangeable between any two consoles. So be sure on which console you want to get your card redeemed.
Cost of 1000 Wii points card is $10 in U.S.A and 2000 card costs $20. So basically you are getting 100 points for every dollar you spend. Other points card available are: 3000, 4000 , 5000 and 7500.
What you can buy with Nintendo points?
Points are used to buy games and other content through its official online channel. You can also buy virtual console if you want to play old games. Millions of Wii games are already downloaded by millions of gamers worldwide so don't be shy to download your game. This credit system is tried and tested by real pro gamers all over the world and all of them happily recommend it to others. There is no problem ever reported by any gamers so you can say its a very safe system. Once you brought your game don't worry about losing because even if somehow your game gets deleted from Wii console you will be able to download it for free next time.