Basically, the AR defines the shape of a given wing. Two wings (or any kind of surface) may have the same area and differ by their AR.

Here are some typical ranges for AR:
Jets (stunt planes): 3 - 5
Sport / pattern planes: 5 - 8
Sailplanes: 15 - 30

High AR values (greater span and shorter chord) are often chosen for slower airplanes, because these airplanes are more affected by a drag generated by the wing tip vortex (vortex-induced drag), this type of drag reduces as AR increases.
On the other hand, high AR models are slow in roll, and a low AR model may present a wider usable range of angles of attack and lower drag at high speeds.