As 'Hamtaro' enters the Clubhouse, Boss is in need of help. He asks you for a name and once you give him it, he tells you that he needs all the ham-hams back at the Clubhouse. Before you are to go out and look for them, he teaches you four basic Ham-Chat words, and scribbles down some others.
You are then sent on your merry way in search of the hamsters. You are able to explore several locations in which two to three ham-hams are in.
The player (Hamtaro) is controlled by the D-pad, and holding down the B button allows the player to run. Pressing A will open a dialogue box with different Ham-chat words depending on context. For instance, standing next to a sunflower seed and selecting "hif-hif" will pick up the seed, while standing next to an NPC and selecting "Hamha" will open a dialogue with the character. Different options will yield different outcomes, rolling (Tack-Q) into NPCs will usually elicit a negative response. Objects in the world can be interacted with, like climbing trees and pulling drawers open.
Source: Wikipedia, "Hamtaro: Ham-Hams Unite!", available under the CC-BY-SA License.