The Tokyo District Court placed an injunction on the Chinese cart manufacturers who make the R4 today, making it illegal to sell the cartridge in Japan. It has already disappeared from shops and Nintendo, Capcom, and Square Enix have all come forward with their support. These companies haven't ceased their efforts yet, however, as they've vowed to continue pursuing those responsible for similar devices.
R4 carts, along with other flash carts, allow users to run homebrew (and pirated) material on their DS or DS Lite (no dice on the upcoming DSi, folks). If this is all still over your head, then check out our guide to homebrew on the DS right here.