Nintendo uses a combination of symmetric and asymmetric cryptography to protect their discs from analysis. This is done through the use of SHA-1 hashes, title keys and a signature.
Wii games are encrypted, and it is possible to decrypt them with a few different tools. These tools are easy to use and offer a wide range of assets.
One tool is called Disc Extractor. This program can read a game's disk and decrypt the files within it.
Another tool is Wii Backup Manager, which allows you to transfer a game's files to an external USB drive or SD card. This will then allow you to play them using Homebrew Channel on your Wii.
There is also a tool called WiiCrypto, which can be used to decrypt WBFS and ISO files. This program can be downloaded for free and is very easy to use.
A recent Slashdot article revealed that Nintendo has used a combination of encryption and signing on Wii games. This is because they want to prevent hackers from changing data and exploiting bugs like the buffer overflow discovered in The Legend of Zelda: Twilight Princess.
A game disc is divided into partitions, each of which contains a title and its contents (which are usually one or two "contents" per title). Each partition has its own unique signature.
The titles and their contents are signed by using a combination of SHA-1 hashes and the asymmetric private key that Nintendo uses to sign. This is to prevent the hackers from modifying the signature to trick the console into accepting it.
All of this is to protect Nintendo’s intellectual property and to avoid the dilution of their trademarks and goodwill. In the case of UK re-sellers, it was ruled that they infringed Nintendo’s trade marks by selling circumvention devices.