Effective range of Wireless Router (dBi) antennas

Have you ever wondered what the antennas specification on wireless router meant ? (3dBi, 4dBi, 5dBi ....... ).

I was shopping for a wireless adapter they tend to have many configuration for antennas.
Amazon Alfa wireless usb adapter

Choose a antenna that meets your requirement.

9dBi dipole (1000m)==> signal far at the same horizontal elevation as the transmission point
7dbi dipole (620m)==> offers an optimal balance between range and elevation.
5dbi dipole (500m)==> over a couple of levels of a building
2dbi dipole (300m)==> If you are in a building and need to cover the rooms around you

More information if you are interested.
http://www.liveport.com/wireless-antenna-properties