Tom Nook Point System
From Nookipedia, the Animal Crossing wiki
The Tom Nook Point System, (renamed Nook's Point-Tracking Machine in City Folk) is a small machine that can be found in Tom Nook's store in Animal Crossing: Wild World and Animal Crossing: City Folk. Its purpose is to record the number of Bells spent by the player in the store and to encourage spending by offering discounts and rewards depending on the amount of Bells spent.
For each 100 Bells spent, the player earns 1 'Nook Point'. Points are not cumulative; any extra Bells from a purchase that are not converted into Nook Points are lost and are not carried on to the next purchase. For example, an item bought for 880 Bells equals 8 points, not 8.8 points.
Function in Wild World
A new addition in Wild World, the Tom Nook Point System is a small rectangular machine that is located lower left in Nook's Cranny, upper right in Nook 'n' Go, and lower right in Nookway and Nookington's. Upon interaction, the Tom Nook Point System allows players to view their point total, see their status, or get general help regarding the machine.
Rewards are received from letters sent by the system itself.
Function in City Folk
Aside from being changed to a green color and having its name changed to the Nook's Point-Tracking Machine, there are two notable additions to the TNPS in City Folk. The player can claim three free 'visitor points' each day (five if this is done in another town), and there are Nintendo-themed rewards available in exchange for points. After the player purchases a reward, the number of points remaining to spend on items is lowered, but not the Player's overall points. This means that the player will remain on a certain points status, even if his or her points balance is below the required number. Only one reward can be purchased per day.
All rewards from Wild World can also be obtained in City Folk; Points status' are also the same.
Once a month (on average), in Animal Crossing: Wild World and Animal Crossing: City Folk, there will be a 'Point Special' day. It will always be on a weekend day with no event taking place, and will be announced on the Bulletin Board three days in advance. During the day, the player will receive five times the normal number of points for purchases both in-store and via the catalog, allowing them to increase their membership level more easily. In Animal Crossing: Wild World, a glitch prevents Point Specials from occurring until a second human character is created in town.