The Town Map in Animal Crossing, with acre designation visible.
An acre is an american unit of measurement that describes the length and width of a square of land. Each acre is composed of an area of 16x16 squares, totaling 256 squares each. This concept was most prominent prior to Wild World where acres locked camera movement and were used to navigate the town map.
Acres are additionally used for determining random placement of objects at the start of the game as well as perfect town ratings which require the right amount of trees per acre, and flowers per acre.
In subsequent games the concept of acres has gradually diminished, though it was still used in Wild World and City Folk to generate object placement. In some recent games players lay down patterns as a visual means of identifying acre boundaries.
In Animal Forest and Animal Crossing
The initial concept of acres was born in the original N64 and Gamecube versions. There are 30 acres in each town composed of 16x16 squares. Other than controlling the camera placement and motion where a player must wait at the boundaries for the camera to "swing" over to the next acre, they serve other limiting purposes. Villagers cannot be pushed through into adjacent acres and many airborne insects will disappear when they travel outside the acre boundary (this does not apply to Animal Forest e+ where insects continue to travel even outside their original acre).
In each acre one regular tree may contain 100 bells each day, assuming a regular tree exists in this acre. If the villagers are inside the acre there house is in you may be able to go inside without them in there.
Wild World
Acres still exist in Wild World, but are not directly visible as the camera is free and does not stick to single acre areas. The map has been changed since the previous games, and acres are not defined as they were in the past. Wild World features 16 acres, for a total of 4096 squares over the entire town.
In each acre there is at least one regular tree that gives out 100 bells daily when shaken, assuming a regular tree exists in that acre.
City Folk
City Folk's acre system is very much like Wild World, with the exception that the town is now made up of 25 acres in a 5x5 grid. The ratio of beehives, furniture and bells per acre is identical to Animal Crossing (GCN).
New Leaf
There are 20 acres in New Leaf, in a 4x5 arrangement. Unlike in previous games, perfect town status is not determined by ratio of flowers and trees per acre and now follows the total number of flora across the entire town. As houses and Public Works Projects can be placed anywhere as long as they are not within one space of an adjacent structure, tree or rock, the use of acres in determining object placement is obsolete.