According to the back of the Nintendog case:
"Give a puppy a new home inside your NINTENDO DS! Your favorite breed is waiting, so take one home with you, then care for & train it using the Touch Screen Microphone".
DD11 & DS7 love it. The player gets to take care of dogs. I always here DD11 saying into the microphone "SIT, SIT". The barking sounds sooo real too. Somehow the player can play alongside another player with their Nintendogs also.